hl is short for "host language" and sets the language of the Google Search interface — and, more importantly for SEO, it biases which language of pages and snippets Google surfaces.
It is not geography. hl does not move you to another place; that is gl and uule. In a bilingual market such as Quebec or Ottawa–Gatineau, holding the location fixed and flipping only hl between en and fr can change which businesses, snippets, and answers appear.
See how hl changes real results: compare Google from any two locations.
Deeper read: hl=en vs hl=fr: how language flips Google results in Canada