The mode itself is called Danger Zone, and comes with Solo, Duo and Trio modes. The total number of players in a match is 16 for solo games, and 18 for team games. Because of the smaller player count, each game should last about 10 minutes.
Weapons have the same damage and behaviour as the original game, with a few extras thrown in for variety’s sake. Player survivability is improved slightly, with health being pushed up to 120. You can also bring over skins from the main game into this mode. Naturally, the launch of this mode also comes with 17 new seasonal skins. Since CS:GO has also gone free-to-play with the addition of this mode, you’re proabably wondering; what happens to those who have bought the game? Well, you’ll be awarded “Prime” status, which makes you “eligible to receive Prime-exclusive souvenir items, item drops, and weapon cases,” according to the game’s Steam page. You can still buy the game and get the Prime status if you so desire. (Source: Valve [1], [2])
