You can't knock on doors in the vanilla version of Skyrim. Simply Knock changes that.
This mod allows you to knock on locked doors in the game's various cities and towns. If someone is inside, they'll come and answer the door. This goes for homes and the businesses located in each place.
It's awesome for those looking for a place to sleep at night or those casing a house or business for a robbery. For more, take a look at the video below.