An orbit is when one object in the universe (for example a planet or a star) goes round another one without touching it.
If, say, a spaceship starts to fly past the Earth, the momentum wants it go in a straight line, but the gravity tries to pull in down to the surface. If the spaceship is going very fast, it will fly past anyway, and if it is going quite slowly, the gravity will win and it will crash into the Earth. But, often the momentum and the gravity will balance and the spaceship will go round and round in an orbit.
You might want to have a look at this demonstration to see more clearly.
A lot of orbits are circular, but some are stretched out to make elliptical orbits.