However, "on" is a part of many common idioms. Here are a few examples:
* On the ball: Being alert, attentive, and ready to act.
* On the fence: Hesitant, undecided, or neutral on an issue.
* On thin ice: In a precarious situation, risking trouble or danger.
* On cloud nine: Extremely happy or excited.
* On the same page: Having the same understanding or agreement.
* On the verge of: About to happen or occur.
* On the lookout for: Searching or watching for something.
* On the house: Given free of charge.
* On the mend: Improving in health or condition.
* On the run: Fleeing from the law or danger.
The meaning of the idiom will depend on the specific words used with "on".
If you provide the context or full phrase you are interested in, I can help you understand the specific meaning of the idiom.