What does the idiom low mean?

The idiom "low" doesn't exist as a stand-alone idiom. It's likely you're thinking of an idiom that includes the word "low," such as:

* Low-hanging fruit: This idiom means something that is easy to achieve or obtain.

* Low blow: This idiom refers to an unfair or unkind comment or action.

* Low key: This idiom means something that is understated or subtle.

* Low-grade: This idiom means something that is of poor quality or inferior.

If you can provide more context about the phrase you're trying to understand, I can help you decipher its meaning more accurately.