Most AR-15 trigger guards have a gap near the pistol grip. When the top of your middle finger on your firing hand rubs against the gap, the metal edges of the gap scrape your finger painfully.
Some people stuff a foam earplug in there temporarily to reduce the issue. This is mentioned in the book Green Eyes and Black Rifles. I found it would inevitably fall out after a while.
There's also a product called the Gapper which is a foam piece specifically shaped to fit the gap.
A reviewer for the product on MidwayUSA mentioned that you can accomplish the same goal by using some hot glue from a hot glue gun in the gap to create your own gapper. I tried it and found it works great. The solid glue stays in the gap very well, and is not permanent; if you want you can actually remove it whole and use it in a different rifle.