It is time for something exciting, interesting, challenging, risky, etc. to begin. "It's showtime," he murmured, as he entered the ring for the boxing match. OK, guys, it's showtime! Let's show the world just how powerful our new device is!
See also: showtime
exclam. (It’s) time to start! (Said of beginning anything exciting or challenging.) Are you ready for action? Okay. It’s showtime!