A roller coaster loop-the-loop is a sort of centrifuge. As you approach the loop, your velocity is straight ahead of you. But the track keeps the coaster car, and therefore your body, from traveling along this straight path. The car starts to turn upward and your velocity is trying to carry you straight ahead. The force of your inertia pushes you into the car floor. This creates a sort of false gravity pulling you toward the bottom of the car when you're upside down. You need a safety harness for security, but in most loop-the-loops, you would stay in the car whether you had a harness or not.
As you move around the loop, the net force acting on your body is constantly changing. At the very bottom of the loop, the acceleration force is pushing you down in the same direction as gravity. Since both forces push you in the same direction, you feel especially heavy at this point. As you move straight up the loop, gravity is pulling you into the back of your seat while the acceleration force is pushing you into the floor. You feel the gravity pulling you into your seat, but (if your eyes are still open) you can see that the ground is no longer where it should be.
At the top of the loop, when you're completely upside down, gravity is pulling you out of your seat and toward the ground, but the stronger acceleration force is pushing you into your seat and toward the sky. Since the two forces pushing you in opposite directions are nearly equal, your body might feel very light. It depends on how fast the car is going and how tight the loop is. As you come out of the loop and level out, you become heavy again.
The loop-the-loop is amazing because it crams so much into such a short length of track. The varying forces put your body through a whole range of sensations in a matter of seconds. While these forces are shaking up all the parts of your body, your eyes see the entire world flip upside down. To many coaster riders, this moment at the top of the loop, when you're light as a feather and all you can see is the sky, is the best part of the whole ride.