Because of Earth's gravity and weight, it actually made it seem as if space and time was "curved" or warped, the theory said.
One way to think about space-time is as a large fishing net.
Stretched out flat, it is straight and regular. When a weight is put in it, it bends. When that weight is spinning, it also twists the net.
So, the probe will look out for this warping, which has not been measured before.
The quartz ping-pong balls in the probe are sealed in a vacuum and are actually gyroscopes which will help the scientists test the theories.
It's expected the tests will prove Einstein to be right. But if his theory turned out to be wrong, lots of things that we know about space would have to be re-thought.