One of the great basics of D&D is that a group of players is on an adventure together. It's the Dungeon Master (DM) who guides them through this adventure. To do this, the DM has to know not only what is possible for player characters, monsters and non-player characters – the other characters you meet and interact with, who are usually played by the DM – but also everything there is to know about the game itself.
Armed with guides like the Dungeon Master's Guide and the Monster Manual, the game module (a write-up of the campaign, which shows the layout of the environment with hazards clearly noted and described) and a set of dice, the DM presides over the game with a nearly omniscient knowledge. It is the DM's job to keep the game going smoothly by describing the environment, asking players what they want to do and then determining the outcome of the events that follow.
Say, for example, you're in a group of characters on a campaign in a dungeon. The DM may tell you something like, "You are in a long, dark corridor. You can see a faint light at one end. To your right is a 10-foot-by-10-foot door. It is locked. Do you want to try to pick the lock or continue down the corridor toward the light?" You and your party decide to have the thief in your group try to pick the lock, so the DM has the thief roll to see if the action is successful. Since the thief has a high dexterity score and only has to roll a low number or higher, he is successful and the door opens.
Unbeknownst to the player (but not to the DM, since he has the game module), on the other side of the door is a neutrally-aligned gelatinous cube, which oozes toward your party. By rolling on behalf of the monsters in the game, it is up to the DM to determine if the gelatinous cube is successful in its mindless attack, absorbing and digesting everything in its path, including the bodies of hapless victims.
A roll of the dice determines if the thief moves in time to avoid the gelatinous cube; if he hasn't, the DM will then roll to determine how much damage the cube deals. Say the thief has 7 hit points. The damage a gelatinous cube causes calls for the DM to roll 2d4 [source: Hack & Slash]. If, in this case, that number totals 7 or more, the thief is dead. He has been digested alive by the gelatinous cube.
But what is the point of all this? Read on to learn strategy.