Summer admissions?

Let’s say a school can’t offer me fall or spring admission. They offer me summer admission. So does that mean the summer of graduating high school? Or the summer after fall and spring is over? And if you enter as a summer applicant do you always do classes in the summer (and fall I’m assuming) all 4 years? Or do you go summer and fall only freshman year? Do you have to reapply to the fall semester again as a sophomore? I’m confused?? Help!!!