This is the Ticker Symbol of the security, an easily memorable identifier used to trade the security on the stock exchange. It is usually, though not always, between 2 to 4 characters long and either alphabetical (AAPL) or numeric (888).
Ticker Symbols will always be unique on a single stock exchange but the same Ticker can be used for a different stock on a different exchange. e.g. “AFC” is used for Arsenal Football Club on the ISDX exchange, but for AFC Energy on the London Stock Exchange. Similarly “VOD” is used both for Vodafone on the London Stock Exchange and for its ADR on the US Nasdaq.
Ticker Symbols are named after the old ticker tape machines that would print out quotes on strips of tape before electronic display terminals became prominent.