module specifier in es6 import and export

I'm confused about what the module specifier in these statements refer to:

export {bar} from "foo";

import {bar} from "foo";

What does "foo" refer to? It can't be a file since it would be something like "./foo". If it's not a file, I assume it's an ID. How is the ID defined?

I'm exporting from a js file, but the import would be part of an inline html script (type="module") in the firefox browser.

The browser version (and browser settings) have been verified to work with es6 modules.

Thanks in advance.