What does it mean when a cat drools but it's clear and doesn't have trouble eating?

The first thing comes to mind is that there might be something wrong with its mouth (gum disease or a broken tooth can cause extreme pain to a cat). However, cats are very good at hiding pain, so whereas your cat is still eating, the drooling indicates there might be something wrong, even though the cat doesn't "seem" to be in discomfort.

It is highly recommended to take the cat to the vet.