It's not just for food; it can also represent a strong attraction or desire when you see a very attractive person or a coveted item. It's used to mean "so good I'm mesmerized."

It originates from scenes in cartoons and anime where a character drools at the sight of delicious food or an attractive person. Sometimes, it's also used humorously to exaggerate a "dazed" state, as if you're so tired you're out of it.