In Dungeons and Dragons there is something called a skill challenge, which involves rolling some dice (no surprise there).
An example of a skill challenge: Skill challenge DC 25 thievery. Roll a d20* and add your thievery bonus (let's say 11) and check if the result matches or goes over 25 take note and roll again (next turn).
As soon as you have five rolls under 25 you fail. As soon as you have 2 rolls over or equal to 25 you succeed in the skill challenge.
How do I calculate the odds of being successful in a skill challenge?
With the numbers I provided I need at least a 14 to match the DC 25. That means that I have $7/20$ odds of matching (or going over) the DC (and $13/20$ for the opposite).
If I only had to match the DC once the math would be easy:
$(13/20)^5$ is the probability of failing, so $1-(13/20)^5$ is the probability of succeeding.
* d20 = 20 sided die, numbered 1-20.