As you work with git a lot of garbage gets accumulated in your .git folder such as file revisions and unreachable objects.
On large repositories and long running projects this negatively affects both operating performance and disk space utilization.
To clean up the garbage git provides a command:
This will not impact the remote repository and will only optimize the local copy so it is safe to run on any git repo you might have. (In fact this operation is already run for you automatically after some git commands that leave behind too many loose objects)
If you want a deeper cleaning (which you should every few hunderd changesets or so), run:
git gc --aggressive
This will take longer but will provide better results.
To learn more:
git help gc