To slap someone is to hit them with the palm of your open hand. It's not nice to slap your sister, no matter how mad you are.
A smack or open-handed blow is a slap, and you can also describe the sound it makes—or a similar sound—as a slap. You could describe the slap of water against the side of your rowboat, or say that your mom slaps her magazine against the table when she's done reading. Slap is also an informal adverb, meaning "immediately" or "directly:" "Sneaking out of class, he ran slap into the principal."