Gravity boots are used to maintain speed on the ground and fall faster. When activated, there will be a stream of blue flowing from your boots. Also, when you hold down spacebar, you will be able to charge up a jump.
To activate the gravity boots just hold shift (default) which will make you fall faster and when you reach the ground you will be able to slide around without actually doing anything.
To use the correctly, you must first have a run up or be flying. While running/flying, simply hold down shift and you will activate the boots. Gravity boots are usually used to move from one side of a map to the other or for falling faster. They can also be used to jump super high.