The idiom get the ball rolling simply means to get busy and keep busy to get something done in a short time or to get something finished in time.
For example let's get the ball rolling and get this house built which means to keep working as much as you can and not stop working unless necessary.
The expression or idiom get the ball rolling originated from a ball game where it's important to keep the ball rolling in the game.
So in short get the ball rolling essentially means to get busy and get something done.