In the US, schools are divided into divisions for the purpose of making athletic competitions more fair. The largest schools are Division I. Because the larger schools have a larger pool of students to form their team from, it's likely that their team will be better. The Division I teams are the ones that get the most attention as their teams are supposed to be better. They also generally have more alumni, money, etc. Because of that, the best athletes try to go to those schools which increases the effect.