What does the idiom is on mean?

The idiom "is on" doesn't exist as a standalone phrase. It's likely you're thinking of a phrase that includes "on" as part of a larger idiom, such as:

* "Is on the ball": This means someone is alert, attentive, and ready to act quickly.

* "Is on the mend": This means someone is recovering from an illness or injury.

* "Is on the same page": This means two or more people understand and agree on something.

* "Is on thin ice": This means someone is in a risky or precarious situation.

* "Is on the clock": This means someone is working and being paid for their time.

If you can provide more context or the full sentence where you heard "is on", I might be able to help you understand the intended meaning.