By playing either Warzone or Halo 5's traditional Arena multiplayer, you'll earn points that can be redeemed for virtual packs of REQ cards. Depending on how many points you want to spend, you can get packs that'll increase your chances of scoring more rare and powerful items. While some of the items you'll get are cosmetic in nature, you'll also get a variety of useful in-game tools that you can equip before you respawn. You can also visit REQ stations scattered around the Warzone areas to power yourself up.
Some cards will be permanently added to your collection, but most of them won't affect in-game combat (the exception beings loadout weapons that you spawn with). The real game-changers are the single-use weapons, armor and vehicles you can get your hands on; during one of the Warzone matches I played, someone on the opposing team managed to unlock some sort of flying warplane that was an absolute game-changer.
Fortunately, even if you have a huge stockpile of powerful cards, 343 Industries has put in a cooldown system to keep you from being near-invincible every time you respawn. Each card has an associated "level" that your team will need to achieve before you can use it. And the higher the level card you use, the longer you'll have to wait before you can use a similarly powerful tool.
If you're worried about getting gouged with in-game purchases for the best gear, 343 Industries notes that "everything" can be earned through the game's REQ points system. It seems that certain items won't be restricted to those who want to use real-world dollars here, but spending your own cash could give you a big leg up on the competition.
The good news is that the REQ system is just one part of what was overall the most fun I had playing Halo 5: Guardians during the private press demo Microsoft hosted recently. The madness of a 12-on-12 match, the unpredictability that the REQ system brings to the game, the combo of AI and human adversaries and the variety of different goals beyond just "kill as much as possible" made Warzone a blast -- even though I was clearly outclassed by a number of other players. If Halo 5's single-player campaign is better with friends, Warzone really made me want to find a whole squad of companions to team up with.