A "Ya Husayn/Hussein" flag behind the Amerikkkan one at the largest (Shia) mosque in North America (Dearborn, MI, go figure)

For context, Imam Husayn/Hussein has big significance as a martyr for Shia Muslims, who annually commemorate his death during the first month of the Islamic calendar. A custom is to raise a red flag with text commemorating him, usually simply saying "O Husayn/Hussein".

If any Westerners know of it, it's likely only from the one flown by the Jamkaran Mosque in Iran, which got dubbed the "flag of revenge" by Western press because aside from the annual religious use, it was used to announce revenge for American or Israeli attacks on its land or leaders.