An object at rest will stay at rest unless something forces it to move. An object will stay in motion, unless something forces it to stop.

Examples:

1.) If I have a rock on a table it will stay on that table(at rest) unless I, some other force moves it.

2.) If I throw a ball it should keep on going. It doesn't though. It falls to the ground because the Earth's gravity is keeping it from moving. If I throw the same ball in the vacuum of space(no air, no gravity), the ball will go forever until another force makes it stop (Like it coming near a large planet or star).

Newton's First Law helps us understand how an orbit works.

How Orbits Work The drawings below simplify the physics of orbiting Earth. We see Earth with a huge, tall mountain rising from it. The mountain, as Isaac Newton first envisioned, has a cannon at the top. When the cannon is fired, the cannonball follows an arc(or curve), falling because of Earth's gravity, and it hits Earth some distance away from the mountain. If we put more gunpowder in the cannon, the next time it's fired, the cannonball goes halfway around the planet before it hits the ground. With still more gunpowder, the cannonball goes so far that it falls on the same curve of the Earth's surface and never touches down at all. It falls completely around Earth. It has achieved orbit.