In fighting games, a true combo is a combo (set of attacks used in sequence) that can't be escaped or interrupted if executed correctly. Essentially, if a combo is true, a successful hit 1 guarantees a successful hit 2.
Player 1: *kills player 2*
Player 2: man, I totally could have gotten out of that.
Player 1: No way, shine into up smash is a true combo against Peach
Player 2. Wow. Fox is broken