Missions are graded depending on completion time, and the fulfilling of optional objectives, and each one may be replayed at any time for a better score. If you fail a part of a mission three times, you also get the option to skip at in exchange for a lowered mission grade. While some may lament this "casualization" of the series, I consider it one of the best things to come to GTA. Open world games regularly suffer from "that one mission" syndrome, that one thing that hamstrings and gets in the way, or a mission type that drives them wild. Being able to sidestep that one infuriating helicopter sequence or hated street race is a nice security option -- and nobody's forcing you to take it.