What is a Heelflip? A Heelflip is where a skater ollies (or jumps with his or her board) into the air, and flicks the skateboard his his or her heel so that it spins in the air along the nose-tail axis, with the heel-edge coming up first, while staying parallel to the ground. The board spins once, and the skater lands on the skateboard when the wheels are facing the ground again, and rides away. Read more about the definition of Heelflip in the Skateboarding Dictionary.
If you are brand new to skateboarding, you'll first need to learn to ollie before learning to heelflip (read Learn How to Ollie). Plus, you may want to take some time getting used to riding your skateboard (read Just Starting Out Skateboarding) before learning to ollie, kickflip or heelflip. Of course, it is totally up to you if you are aggressive and want to learn to heelflip on your skateboard before learning how to ride, thats up to you!
Make sure you read all of these instructions before you try to heelflip. Once you are familiar with them, go skate!