An automatic bid in basketball is one of the 32 spots in March Madness that are automatically awarded to the teams who have won their respective conference tournaments, which are held at the end of the regular season. (Note that there are 32 Division I basketball conferences in the United States, and each of the 351 Division I basketball teams must belong to one.) With an automatic bid, you have qualified for the March Madness Tournament, college basketball's postseason to determine the best team in Division 1 basketball. There are a total of 68 teams who can qualify for the March Madness Tournament, either with an automatic bid or an at-large bid.
Automatic bids are decided by conference tournaments, where every team in the conference us given a seed based on their record in the regular season, and a tournament proceeds to determine the conference champions. The conference champion is awarded an automatic bid. Teams with a higher seed heading into the conference tournament are awarded byes through one or two rounds. This is critical, because conference tournaments start and finish in less than a week, so teams with byes have more time to rest and prepare.
Every year, there are unlikely teams who end up winning their conference tournament and acquiring an automatic bid. One example is the 2011 Uconn Huskies. They went into the Big East Tournament as the 9 seed, and ended up winning 5 games to become the Big East champions. And their improbable run continued into March Madness, where they beat opponent after opponent to end up winning the National Championship over the Butler Bulldogs.
The 2017 Michigan Wolverines were an 8 seed heading into the Big Ten Tournament, and the biggest obstacle they had to get past wasn't on the basketball court. The plane ride for the Wolverines to Washington DC was a complete disaster, where the plane ended up sliding off the runway. The team was shaken up and arrived just a couple hours before tip-off. They also were forced to play their first game in their practice jerseys due to the plane troubles. Even through all of that, they took care of their first game against Illinois and ended up winning the Big Ten Tournament.