The younger generation is tired of using Millennials as a curse word against them. So they created a meme to counter: "OK Boomer". The phrase is meant to express the fatigue of older people, born during the Baby Boomer era, until about 1964, when they are instructive, criticizing and condescending.
Especially the very young users of the platform "Tik Tok" use this sentence against older people who are especially conservative.
In New Zealand, the meme was of particular importance because the 25-year-old MP, ChlOe Swarbrick, gave a speech on climate change and remarked that it is her generation and their descendants that will suffer the crisis. When it came to intermittent interjections calling her age, she responded with the memo, "OK, Boomer."