it is a slang used when some old person or someone from Gen X, or maybe even a Karen tell millennialswhat to doand what not to and Gen Z reply with "Okay 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