Yeah, basically a trigger is something that might cause people to have PTSD-esque flashbacks, or prompt self-destructive behavior. Some people who are recovering from anorexia, for example, really do not need to be reading fics about eating disorders.

I am not at all opposed to warning for obviously traumatic things, like rape or self-harm. But it's gotten to the point where people are trying to get their squicks added to the list of triggers, which really makes a mockery of what triggers are all about. Note to fandom: just because it doesn't turn you on, doesn't make it triggering.