Team Builder works by having players select their champion, role and position for a match, and then attempts to match them up with other players using Team Builder until a balanced team can be formed. Theoretically, this will lead to less fighting about who plays who, goes where and does what. The Team Builder queue will be separate from others, and will only be available for unranked 5v5 matches.
Team Builder won't be sticking around forever. League players will have 48 hours from the time the beta goes live in their region until Riot closes off access. The beta will be a staggered launch, starting March 3 on the RU and EUNE servers. To see when your region is live and learn more about Team Builder, you can check out the official FAQ page.