An orbit is the path an object in space travels around another. Lots of objects in the Universe orbit others. The Moon orbits the Earth. The Earth orbits the Sun. Did you know the Sun orbits the centre of the Milky Way?
An orbit is a curved path. A lot of orbits are shaped like a circle. Some orbits are more oval shaped. We call these elliptical orbits.
Orbits happen because of gravity and something called momentum. The Moon's momentum wants to carry it off into space in a straight line. The Earth's gravity pulls the Moon back towards the Earth. The constant tug of war between these forces creates a curved path. The Moon orbits the Earth because the gravity and momentum balance out.
The gravity and momentum must balance out to create an orbit. If the momentum is too large, the object speeds past. If the momentum is too small, the gravity pulls it down, and it crashes.
Want to know how spacecraft and satellites orbit the Earth? Have a look at this demonstration.