The Nudge/set items dialog is another extremely useful feature within Reaper. You can open it with N.

As the name suggests, it allows you to either nudge the position of an item or set it to a specific location. Nudging the position is extremely useful however you can do far more than this. For example, you can nudge the right trim set the position of an item, or duplicate an item a specific number of times placing each item on a beat, or every second