'A catch phrase' commonly used to express 'enthusiastic support' for a particular 'subject/or subject matter'.
(With more of an emphasis placed on the word 'that's')
*acronym/abbreviated form: NTWITA or TWITA 'That's What I'm Talking About'.
A guy just opened his first pay check- and he discovered, that the amount of money enclosed, was a lot more than he was expecting!
-> so he say's (while nodding his head in approval)~ 'Now that's what i'm talking about' !