A heelflip is a trick similar to an ollie, but is somewhat more complicated. The skateboarder ollies off the ground, and once the board is in the air, flips the skateboard with their heel, so that the skateboard rotates around completely. Then the skateboarder lands on the board again and continues on. (The heelflip is much like a kickflip, but the board is rotating in the opposite direction.)
1) Set up your feet the same way you do for an ollie
2) Ollie off the ground
3) As soon as the skateboard is in the air, kick out your top foot, so that your heel pushes down the edge of the skateboard. (If you ride regular, you will be pushing the right side of the board down with your left heel. Similarly, goofy riders will be pushing down the left side of the board, and rotating it around to the left.) Try to keep your feet free from the spinning skateboard. The board should rotate around once, without your feet coming in contact with it until it comes back around to the normal position.
4) Once the board is wheels-down again, un-crouch your legs so that your feet come to rest on the skateboard. Then land on the ground with your feet in the same position.
Try these tricks next time you hit the skate park: Power Slide; Frontside 50/50; Tail Stomp.