Everybody has heard about ASCII or ASCII-Code before.
Again an abbrevation. ASCII is the short form for: American Standard Code for Information Interchange.
ASCII is a so called 7-Bit-Code for letters, numbers and special control-characters, first published in 1963. It matches with its printable characters the American typewriter keyboard.
Control-characters are characters for tabs, pilcrow etc.
If somebody tells you to send him an ASCII-text, he usually means a completely unformatted text … a plain text.