This series of photos shows how you can use a belt loop to act as an anchor, limiting the travel of your holster along your belt.
This is beneficial because it keeps the pistol where you want it should you need to draw it from the holster yet it still allows for limited travel so you can adjust on the fly.
I hope this helps you out, it has served me well and if you are new to carrying may not have occurred to you.
Steepshot