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 on the betas tab. This will require you to re-download the current public files.