Call signs are a form of telephony designator, a sort of nickname for radio communications. Airline call signs help to identify a plane to control towers and other aircraft, similar to the use of airline designators.
Airline call signs may be just their company name, such as “AEROMEXICO” for Aeroméxico, but they can also be a bit more fun, such as “SHAMROCK” for Ireland’s Aer Lingus. When written, it is common to capitalize the entire call sign word.
For a table of compiled call signs, see here: Airlines Official Call Signs Table