"You know Ben, I'm glad I killed your wife"

Someone please explain to a peasant like me?
- Those two are characters from Star Trek: The Next Generation and Star Trek: Deep Space 9. Captain Jean-Luc Picard and Captain Benjamin Sisko. A couple of things are required for the joke to make sense.
  The first is that Picard was previously assimilated by a cybernetic species called Borg. They kidnap individuals and plug them into their hivemind, erasing all traces of individuality while also gaining every piece of information that the individual knows. When the Borg assimilated Picard, they used his information to launch an attack on the Federation. A massive battle ensued at a star system called Wolf 359 where a large number of Starfleet vessels were utterly destroyed by the Borg, including the ships of Sisko and his wife. Sisko survived. His wife did not. Picard is eventually retrieved from the Borg and 'unassimilated', carrying on with his life.
  Many years later, Picard is abducted by a group of aliens called 'Cardassians'. In an attempt to get information from Picard, they torture him in a variety of ways but one in particular is shining four lights in his face and insisting there are actually 5. It's an attempt to break Picard mentally and make him question everything. After being brought to a breaking point, Picard is saved. As he's being taken away, Picard turns to his torturer and screams "THERE ARE FOUR LIGHTS" into his face as a sign of his defiance and showing that the torturer did not break him. Privately, Picard did say to his counselor that for a moment he could see five lights.
  The joke involves Sisko making fun of Picards torture because Sisko partially (if not fully, in some ways) blames Picard for the death of his wife. The title joke then throws the joke back in Siskos face saying "yeah yeah, lights but your wife is dead lol".
  
  Fantastic! Thanks for this awesome context and explanation!
  Edit: thank you all for delivering!
- Thank you!
- When Picard was captured by the Kardassians he was tortured and they attempted to brainwash him. He was shown 4 bright lights and continuously told there were 5 (I think 5) of he said there were 4. The sequence ends with a final over the top amount of torture and he the last thing he said before being returned to federation was there are only 4 lights (I think). Later he is talking to someone and says he actually saw 5 the last time I believe.
  
  Thank you!
- !Picard, as Locutus of Borg, killed Sisko's wife at the battle of Wolf 359. The four lights reference is related to an episode where Picard is tortured and to price his well is broken the cardiacs want him to say there are 4 lights when there were actually 5. Picard never breaks during his torture and quite emotionally yells "there are five lights"!<

Clearly, there are 5 lights. I don't know how you could be so wrong about this.

This always seemed like the cheapest way to manufacture tension for me.
There are serious core philosophical differences between Sisco and Picard. There are multiple avenues that could have been used to create conflict and tension.
Instead we get Sisco hating Picard because he blamed him for that shit with the Borg. At best the first impression of Sisco is that he's kind of an idiot. If you dig deeper Sisco is a victim blaming asshole who hates Picard for serving as a meat puppet after getting brutally violated by the Borg.
You could have just switched the reason for Sisco to hate Picard to the fact that he had his chance to strike a fatal blow to the Borg and refused for reasons Sisco would see as sanctimonious.
- First, it's Sisko. Not giving you shit, just letting you know.
  Second, I didn't mind it that much but you're right that there were a lot of avenues but Sisko is shown a few times to be insanely emotional in comparison to Picard. JL tends to be a bit more methodical and logical about stuff, almost going down a Vulcan-like path of logic. Sisko, on the other hand, is not really like that. Sure he thinks things out. He will plan shit out. He'll try to be methodical but he's also incredibly prone to just outbursts of emotion. Sisko has a very basic level of control over his emotions. He can keep them in check but often he doesn't. Look at the intro between Picard and Sisko as an example. He loses his shit at Picard because of the Borg stuff and it takes him a while for him to get it in check enough to have a discussion with Picard without lashing out at him.
  He was so upset at Michael Eddington that he goes on a fucking revenge tour with a barely functioning Defiant to try and take him down and that's ignoring the gassing a planet part because one could argue (although I disagree with it) that he was making a logical decision there due to his bit earlier about realizing he needs to play the villain. Sisko realizes that Garak killed the Romulan Ambassador and fucking assaults him in his store, throwing him around and punching him in the face. Speaking of punching people in the face, HE PUNCHED GOD IN THE FACE. I'm sorry, but that's not logical. That is one hell of an emotional decision to be so annoyed you punch God.
  Sisko being set up in that opening with Picard as an emotional hothead honestly feels pretty true to character. While they had differences in philosophy, they were way more different in the sense of who was more emotional than the other. I liked the highlighting of that personally but I can see why one did not.
  
  I’m more disappointed that ‘Movie Picard’ and then ‘Picard show Picard’ abandoned, or at least lost the emotional regulation that enabled him to hold onto, many of the principles that made him so admirable and exasperating.
  It doesn’t seem like Picard in season three of Picard would have had any of the same qualms, or at least his emotional attachments would have overtaken them.
  I wonder what Ben Sisko’s reaction to Picard’s choices in season three would have been. I definitely think he would have called him out, and made Shaw’s critiques look tame.

Picard: I almost thought I saw 5 in storage.