I am a second baseman. I had a few questions about holding a runner?

I play second base.i a trying to figure out what to do when there is a runner on second.people tell me to run to second base and stay there to then when the pitcher delivers run back to my position.do I keep running back and forth every pitch when there is a runner on second or do I wait for a sign to move in to hold the runner idk I am getting confused