Learn French: Le lion mange de la viande.

In French, you rarely use a noun without an article. Because meat is an uncountable noun (a mass of something, rather than individual items), we use the partitive article de la, which roughly translates to "some". So literally, the sentence says that the lion eats "some" meat.

What happens to de la if I make the sentence negative ("The lion does not eat meat")?

In a negative sentence, the partitive article de la changes to just de. So, "The lion does not eat meat" becomes Le lion ne mange pas de viande. This is a strict rule in French for negative quantities that learners often forget.

If I meant "The lion eats the meat" (referring to a specific piece of meat), how would the sentence change?

You would replace the partitive article de la (meaning "some") with the definite article la (meaning "the"). The sentence would then become Le lion mange la viande.

Can Le lion here mean "Lions" in general, like in a nature documentary?

Yes! In French, the definite article (le, la, les) is used to talk about general categories and species. While you could say Les lions mangent de la viande (Lions eat meat), using the singular Le lion to represent the whole species is very common and natural in French.