People who believe in purgatory believe that there is a place "between" Earth and Heaven, which is just blank--meaning that the soul can neither return to Earth or go to Heaven. Indulgences were meant to be bought in order to assure that the soul would continue on to heaven.
Purgatory is a religious belief that that is where someone might end up if that person behaved good and bad in their lifetime, not enough to merit either heaven or hell. It was like a judge hearing that decided eventually what that persons eternal fate might be. It’s like debating the good vs bad which eventually gives a result. It may take some time from what I hear. The sale of indulgences are either to aid redemption or to credit being evil, or just to expose that that person cannot or is not willing to change.
