Used in forums or similar areas to say that you agree with what the person you're quoting said. Similar to "MTE (My thoughts exactly)" or "I couldn't have said it more eloquently myself" or, more colloquially, "This" - or "ditto", "+1", "seconded", etc.
Stephanie: "I just totally blew that history test! My life is in ruins!"
Denise: "Come on, don't worry so much about it. You know your stuff, and anyway, it's only like 5% of your grade."
Jen: "Yeah, what she said."
Spot the Reference. Used when you've just made an allusion to something (often a part of popular culture such as a quote from a movie) and you're inviting the audience to show that they recognise what you're alluding to.