Sentence examples for it is on from inspiring English sources

"it is on" is a correct and usable phrase in written English.
It generally means that something is happening or taking place in the present moment. Example: "I can't talk right now, the game is on." This means that the person is currently watching a game or sporting event and cannot talk at the moment. Another example: "You can turn the TV off now, the movie is on." Here, the speaker is telling someone that they can turn off the TV because the movie is currently playing. In both of these examples, "it is on" is used to refer to something that is happening in the present moment.