A statement used to draw attention to a phrase that could be interpreted as sexual innuendo. This interjection is used when someone, usually a female, unintentionally makes a sexual comment. The "he" is typically assumed to be the girl's boyfriend.
The inverse use that's what she said
was made popular by the hit TV show The Office
Scenario: Two girls are getting out of the pool and going to grab towels to dry off...
Girl 1 (while picking up towel): Wow, that's ones really wet!
Girl 2: That's what he said.