CS:GO Beta depots allow the game's developers (and users) to test changes not yet available in the Public version of the game.
When opting into a Beta depot players will be required to download updated files. The download size will vary based on the number of changes. Please note official Valve-matchmaking is not available in Beta depots.
To opt into Beta depots:
To opt out of the Beta and resume playing the current version of the game, select 'NONE' in the drop down box of the BETAS tab. This will prompt another restart and will require you to re-download the current Public files.