In many different sports, an attempt to score points or complete something can't be done without first starting to roll the ball.
This could be a golf ball rolling into a hole. This could be a bowling ball rolling down the alley.
This could be a soccer ball getting moved forward to start a game.
"Get the ball rolling" also applies to "real life" as well.
To "get the ball rolling" means to begin something.
For instance - I got the "ball rolling" on a group project at work by starting to do market research.
Or, I got the "ball rolling" on the restoration of a vehicle by ordering a new bumper for the car.
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"Get the ball rolling" means that you are getting something started.