What is the difference between position and velocity iteration?

I’m student in Japan, and I use PhysX to simulate rigid body dynamics.
So I set a lot of actors in the scene, and set iteration count to them.
But I can’t understand the difference between position iteration and velocity iteration.
I read the API reference, but I can’t understand clearly between them.

If you know the difference between them, please teach me.

Sorry for my poor English.

Changing the velocity iterations will change stability, because it can integrate the velocity and acceleration over a series of timesteps.
Position iterations affect the overlap resolution which can reduce errors generated during position correction.

Hi EmmaEh.
Thank you for reply, and I’m sorry answer is too slow.

I can understand the difference between them for your reply.
Thank you very much!