Gravity keeps your feet on the ground until you choose to fly. You can fly
at any time by pressing the + (plus) key on the numeric keypad. Press the -
(minus) key again to return to the ground and make gravity work again.
"Ground" in this case doesn't need to be the actual world ground level, just
a solid horizontal object below your feet. Note that on laptop computers,
which don't have numeric keypads, the + and - keys are usually
accessible via a combination of a function key and some other key
on the laptop keyboard.
Gravity is turned on or off in a world by the presence or absence of
a ground object.
Note that when you're flying, you'll coast more and won't have as much drag as you
do on the ground. If you find yourself coasting too far, press the "5" key in the
center of the numeric keypad to come to a complete stop.