An instinct is something you don't need to learn — it happens naturally, without you even thinking about it. Babies cry by instinct, and ducks follow their mother by instinct.
Animals and humans learn a lot of things from other animals and humans. But if there isn't any learning involved, then the behavior is an instinct. Instincts come naturally, like a baby's desire to feed. Some behaviors are a combination of instinct and learned behavior, like language. Others happen without any teaching at all, like the instinct to run when you see a big, hungry looking bear. When you see the word instinct, think natural response.