Look again.
Sometimes, resolving a task is easy. If an adventurer wants to walk across a room and open a door, the DM might just say that the door opens and describe what lies beyond. But the door might be locked, the floor might hide a deadly trap, or some other circumstance might make it challenging for an adventurer to complete a task. In those cases, the DM decides what happens, often relying on the roll of a die to determine the results of an action.
So, in cases where resolving a task is not easy, the DM decides what happens. Often (but not always) the DM will rely on the roll of a die to determine the results. It is entirely within the DM’s power to decide what happens without relying on the roll of a die.
If the DM determines that a roll is necessary, the rules provide instruction as to how to resolve that roll.