No offense intended, but i do notice a lot of the time you act like you know everything about whatever the topic is about. Most people will know a lot more but they just won't say it. And dafuq is 1d20 anyway? >.>
Like kaitoyuuki tryed to tell, it's in RPG (not video games, but in real life) with dices, and in 1d20, the 1 is the number of dice, d means dice, and 20 is the number of faces of the dice (yes, there are many dices, it can also have 3 faces, 4 faces, 5 faces, 6 faces, 8 faces, 10 faces, there are also % dices, 2d10 but one shows only 00, 10, 20, 30, 40... up to 90 and the second tells 0, 1, 2, 3, 4, 5... up to nine, also 12 faces or 100 faces, but 100 faces and 5 faces are never used, cause 5 appears in only ONE specific game and instead of 100 faces we prefer to use % dices) I made this joke cause I go 1 saturday/2 for a whole day to play RPG games, that I played just 2 hours after my post