It's an addon, which means that it's a separate file that you can download and install.

The first thing that you will notice when you start playing the game is that there is a new item in the game, which is called a bed.

You will find it in the room that you can access by clicking the "key" that is on the door.

When you are in the room, you will see a new item on the bed.

If you want to find out what this mod is, you can read the description that is included in the zip file.