ASCII (American Standard Code for Information Interchange) assigns numbers 0–127 to characters. Capital A is 65, lowercase a is 97, the digit '0' is 48.
It only covers English. Anything with accents, Chinese characters, emoji, or even the Euro sign needs a larger encoding, that's what Unicode (UTF-8) is for.
Every Unicode character with a code point under 128 has the same bytes in UTF-8 as it does in ASCII, which is why UTF-8 is backward-compatible with anything that handles ASCII.