When you're just like sitting there or walking somewhere or laying elsewhere, and you're just thinking and then all of a sudden a memory pops into your head and its a bad one, either because its something you were sad about or angry about or worse, embarrassed about. And you tell your brain to fuck off with the memory but it just laughs at you and makes you remember the event harder.
Basically I hate this feeling and brains are bad people. You cannot trust them.