What does the idiom on mean?

The idiom "on" doesn't exist as a stand-alone idiom. It's likely you're thinking of a phrase that includes the word "on" as part of a larger idiom.

Could you please provide more context or the full phrase you're thinking of? For example:

* "On the other hand..."

* "On the same page"

* "On thin ice"

Once you provide the full phrase, I can explain its meaning.