"Baby boomer" is a term used to describe a person who was born between 1946 and 1964.
According to dictionary.com:
"OK boomer"—dismisses a person from that generation (and older people more generally) as out of touch, close-minded, and part of the problem.
The specific slang expression OK boomer took off in early 2019, issued especially from millennials and Generation Z as a reply or reaction to older users (lumped together as baby boomers) who posted content that condescended to younger generations or provoked their sensibilities.
So, IMO, it's a derogatory term from someone in another (younger) age group telling the older person they are out of touch. Just for the record, my generation would never eat Tide pods as a viral challenge ????