In the vanilla version of Fallout 4 you can only wear certain articles of clothing underneath your armor. With this mod installed, you can wear whatever you want.
With Under-Wear installed on your console you're able to wear any clothing or outfits underneath your armor of choice. That's not all.
The mod also allows you to put ballistic weave on any clothing, outfits, or headgear. This includes the dog armor you likely have on Dogmeat.
It's an excellent mod and it's perfect for those that want to frequently change the look of their character.