it is a slang used when some old person or someone from Gen X, or maybe even a Karen tell millennials what to do and what not to and Gen Z reply with "Okay boomer"
OLD GRANDPA: Stop looking at that damn phone. This is why you get sick all the time.
Gen Zs: Ok Boomer
OK boomer is an internet slang phrase used, often in a humorous or ironic manner, to call out or dismiss people that are close-minded or people that don't understand your humour or point. A boomer is often described as an older person with the trouble of understanding newer culture and humour
Person1: "shows meme".
Person2: "That's not funny".
Person1: "Ok Boomer"