ASCII (pronounced ASK-EE) is an American based character encoding system for computers to understand sequences of characters. In other words, it's the low level technology so your computer can know the English alphabet, numbers, and other special characters (128 characters in total).
When the web was first created, it was the standard encoding system for websites (since ASCII was a very common encoding system on a lot of computers). This means characters for other languages would not have worked. Since the invention of the web, a more modern encoding system has been created called UTF-8 which allows for just about every character for every language.
W3Schools has a good reference of the characters in ASCII.