In the NCAA March Madness Basketball Tournament, there are two types of bids: an automatic bid, given to the winner of each conference's champions (of which there are 32), and an at-large bid. The at-large bid is for teams that do not win their tournament but have enough talent to qualify for the tournament anyway. These 36 teams are selected by the Selection Committee on Selection Sunday, who also seed all 68 teams in order to set the field.
There are many ways a team can qualify for an at-large bid. Statistics such as win-loss record, offensive and defensive efficiency, and other factors may lead to a team being selected. In the past, the Committee has used a statistic called RPI, which gives a definitive ranking to each team based on a myriad of factors. The NCAA attempted to introduce a new version of the RPI, the NET rankings, during the 2018-19 season.
Conference strength is a huge factor when deciding the bids. Teams that play in the Big Six conferences (SEC, ACC, Big 10, Big 12, Pac-12, and Big East) are giving priority due to the challenging nature of their conferences, while lesser conferences such as the American, Mountain West, and West Coast have fewer teams that qualify for the at-large bids. Even teams in those conferences that win the automatic bid get seeded lower than many at-large teams due to the nature of the conferences. This is why statistics like RPI and NET are helpful as they level the playing field and allow mid-majors to have a chance to be selected for at-large bids.