It means your client (looks like Jerboa) wants Lemmy API version 0.18 but lemmy.world has not been updated yet. Hang tight, I believe the admins are overloaded with the new user influx.
For accuracy, it’s not just about the admins being busy, 0.18.0 removed the signup captcha, which has been crucial for preventing massive bot signups into instances, some number of instances can’t upgrade without that critical downside. The plan is for the captcha to be reintroduced in 0.18.1, so we’re all waiting for that.
Lemmy.world is not upgrading to 0.18 because they removed captcha, admins are waiting for 0.18.1 in which captcha is being re-implemented, it's discussed here: https://lemmy.world/post/469761