The Support role is to aid and help protect your allies in any situation. Your role is is to shield or heal them, to tanking some damage or sacrificing yourself to keep others alive. You are often giving up the glorious kills in the game to support others, but in the end its usually the support to secure the true MVP award. Support heroes usually lane with the marksman in top lane or at mid lane in order to be able to travel quickly where they are needed.
Support is a type of role that provides some kind of benefit to their team. These benefits provide sustainability through healing or giving shields or deal some form of CC like stunning enemies. They often accompany carry heroes to help them level up faster. They're are recommended to participate in team fights to help teammates in accomplishing objectives and not solo a lane.