I've played it only once, so I can't explain the rules in depth or anything like that, but I can explain what it is in general terms.

Dungeons & Dragons, and other tabletop RPGs, are games built around collaborative storytelling. One player, the Dungeon Master, is in charge of deciding upon the setting of the story (as the name imples it's most conducive to medieval fantasy settings), acting as its narrator, and keeping the story moving forward. The other players create and control the main characters. The outcomes of various events that challenge the main characters, including combat, are determined by a mix of dice rolls, character stats, and Dungeon Master decisions as specified by the D&D rules.