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 gravitational pull of the Earth tries to pull it down to the surface. If the spaceship is going very fast, the momentum is enough for the ship to fly past anyway, but if it is going slowly, the momentum isn't enough and the gravity will win and it will crash into the Earth.
You might want to have a look at this demonstration to see more clearly.
For things to orbit around each other the momentum and the gravity need to balance.
A lot of orbits are almost circular, but some are stretched out to make elliptical orbits.