This applies to servers only, and is used to download a map faster.

Don't know how to make a .bsp into .bz2? Here's how to convert them using 7-zip.
Source from Fr0Z3nR on TF2Maps.net: https://tf2maps.net/threads/tip-how-to-compress-bsp-to-bz2-with-7zip.18108/

Before you compress, you need to make sure your maps custom assets are packed properly. The best programs for this is either Pakrat, or PackBSP. How to pack custom assets is not included in this guide, but should be covered in the programs documentation.

Step 1: Get 7Zip.
If you don't have it, get it here, and install it. If you do have it, skip to step 2.

Step 2:
Go to your tf/maps folder (steam/steamapps/{UserName}/team fortress 2/tf/maps. Here you will see a list of maps that end in .bsp 

Step 3:
Right click on the .bsp of the map you wish to compress. Then click, 7Zip > Add to Archive.

Step 4:
In the window that opens up, click the "archive format" drop-down menu and select the bzip2 file format. Do not adjust anything else. **Do not adjust the name of the file**

Step 5:
Click ok. You now have, beneath the original map file, a .bz2 compressed map file, ready for you or someone else to upload to the game server(s). You can move this file to your dropbox, or use the fancy file hosting services provided by this site. 