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Iran Raises Red Flag Of Revenge
(kashmirobserver.net)
I looked up what this flag is about.
The flag bears the Arabic phrase, “Ya la-Tharat al-Hussein,” which can be translated in English as, “O ye avengers of Hussein.” As Muhammad’s grandson, Hussein is one of the most sacred figures of Shia Islam, in which he is the incarnation of justice. He was killed in the year 680 at the battle of Karbala, in present-day Iraq, by caliph Yazid’s troops.
This battle and Hussein’s martyrdom are the founding elements for Shia Islam, and mark a definitive schism with Sunni Islam. According to certain Iranian Shia traditions, “Ya la-Thara al-Hussein,” should be a rallying cry for believers upon the return of the Hidden Imam, the last of twelve imams recognized in Imamiyyah, the largest branch of Shia Islam.
Source: https://factcheck.afp.com/no-red-flag-revenge-was-not-raised-first-time-iran-mosque
It's flown regularly in the first month of the Islamic calendar. For Shia Muslims, it's a period of mourning dedicated to him. Outside of that, it's on moments like this, meant as a sort of rallying cry.
Wikipedia claims that "it also flown [sic] in response to the COVID-19 pandemic in Iran whenever a milestone of casualties was reached", but I couldn't find sources for it elsewhere.