Cast Magic Missile
Roll D20
3.
Ineffective.
Monster eats you and your adventure ends.
I never actually understood how that game is played.
I've never actually played it, so it's hard to say.
I've played a couple (read: 2) that follow similar systems, though. Essentially, when you create a character you roll stats like charisma, strength, vitatlity, etc that determine bonuses or penalties against whatever your DM/GM decides to throw at you. In most systems you can acquire skills and items that augment these base stats.
Say we were playing Medschool & Gunners, 78th Ed. You come up against AMCAS, a horribly disfigured manbeast from the dark places of dreams. You decide you want to enter your transcripts. As entering transcripts would have an effect on the status of AMCAS, a non-player character, you roll to see if you pull it off. The game master has set an arbitrary number you need to clear in order to successfully enter your coursework. As a character of class "Gunner" you have a +3 bonus to begging for good grades, but a -2 penalty to actually doing work, making data entry a laborsome chore for you. You roll a d20 to see if you are successful.
You roll a 20! Critical success! Not only have you entered your coursework, AMCAS has prewritten your personal statement!
Had you rolled under 20, but over the threshhold value+2, you would've simply entered the data.
Had you rolled under the threshhold value+2, but greater than zero, your internet connection would have failed, thus blocking you from logging in.
Had you rolled a 0, AMCAS would let you enter data, accept your money, and then make you wait 12 weeks after the first submission date to tell you the database with only your information on it had been corrupted, you need to make a new account, pay them more money, and oh, by the way, I don't think you should use those letters.