ASCII is an Acronym for the American Standard Code for Information Interchange.
ASCII assigns numbers (0-128) to represent all the English letters, numbers and symbols, including upper and lower case.
ASCII is used to store most text files in English on computers.
The reason ASCII is included in a hearing loss glossary is that most computer communication is done in ASCII. Since many hard of hearing people use computers for email and instant messaging, they use ASCII even if they don't know it.
It's also of interest because some communication related to hearing loss is done in other codes. For example, TTYs communicate using Baudot.