ASCII, for those readers who don't intimately know the smell of an unventilated 1980s basement, stands for American Standard Code for Information Interchange, or simply put, all the characters in your computer's pre-emoji alphabet. ASCII has existed as long as personal computers have, so before the days of actual graphics, ASCII art was made which turned these symbols into the kind of images you'd expect coming out of a malfunctioning fax machine.
Of course, 1337d00m still keeps the fast-paced gameplay of the original -- something ASCII isn't really built for. If you think that makes it ridiculously harder to play the game, that appears to be the point. Zubovic claims that it's "a parody of difficulty sliders in games," with their random qualifiers for making things artificially easier or harder. 1337d00m's difficulty slider that is basically just a font resizer. The lower the difficulty, the smaller the font and the more detail there is on the screen. Meanwhile, the "Impossible" difficulty doesn't resemble anything other than a random string of keyboard inputs, making it look like the game was rage-quitting on you instead of the other way around.