Meaning: Be functioning (of machines)
Example: The computer IS ON.
Meaning: Take place
Example: The show IS ON for the next three months.
Meaning: Take medication or drugs, especially when they affect the person badly
Example: He IS ON anti-depressants and has become very difficult to please.
Meaning: Be at the top of one’s game, performing very well
Example: He IS really ON right now- three goals in five minutes!