What does the idiom as a mean?

The idiom "as a" is not a complete idiom. It's likely you're thinking of a phrase that begins with "as a" and continues with a specific noun or phrase.

To help me understand what you're looking for, could you please provide more context or the full phrase you're thinking of?

For example:

* "As a rule"

* "As a matter of fact"

* "As a last resort"

Once you give me the complete phrase, I can explain its meaning.