ASCII or American Standard Code for Information Interchange is the numerical representation of a character, letter or symbol. Computers can only understand numbers so they use ASCII.
When computers first came about, different manufacturers used their own codes for their computers. But they soon realised that if computers wanted to share information, they needed to also share a common encoding system for text. ASCII was the first character-encoding scheme to be used as a common standard for all computers.
Letters can be represented in 8-bit binary (base 2) or in ASCII which uses Hexadecimals (Hex or base 16) which is the one that Mark uses to communicate with earth after he finds the ASCII table in Beth's stuff.
There are 128 different characters to chose from in ASCII including upper case letters, lower case letters, punctuation and numbers. Using this table you can write anything in English. To write in other languages that don't use English characters or even to write in emojis computers use 32-bit Unicode which has about 4,294,967,296 options.
EG: 84 104 101 32 77 97 114 116 105 97 110 = The Martian