Difficulty class

The Difficulty Class is a number associated with an action that represents how hard that action is to do. When the game engine checks to see if an action succeeds, it will generate a number between 1 and 20, and add any relevant modifiers to that number. If the resulting number meets or exceeds the difficulty class of the action, the action succeeds. A very common example is when a character must perform a saving throw, so that is what we will use as our example.

Example
Imagine that ArgleBargle, the charming and dashing wizard of some renown (in his own mind, at least), had been too busy rooting through his pockets in search of candy to notice that the rogue in his party had stopped in his tracks and begun gesturing frantically that he has spotted a trap. Marching past his desperate friend, ArgleBargle triumphantly produces a gobstopper, and steps on a pressure plate. A loud "click" is heard, and, with a puzzled expression on his face, ArgleBargle finds himself regarding the dart that now sticks out of his posterior. The game engine then checks to see if ArgleBargle successfully performs a saving throw vs. poison.

This is a mild poison, with a Difficulty Class of 10. The game engine will roll a number between 1-20, and then add ArgleBargle's saving throw modifier to that number. If the resulting number is 10 or above, ArgleBargle will walk on unscathed. If the resulting number is below the poison's Difficulty Class (10 in this case) then ArgleBargle will have failed his saving throw vs poison, and suffer a nasty itch that will last for weeks.