what does "single-click hold" mean, and how would one use it?
If you enable single-click holds, it will allow you to skip the stage of selecting a pick-up location each time you place a hold; it will automatically use your default pick-up location. This is useful (and quicker!) if you usually pick up your holds at the same branch anyway. You can read more about this and learn how to select this setting at http://help.bibliocommons.com/040settings/010singleclick/
I hope this helps!