A ticker symbol is the short code — usually one to five letters — that uniquely identifies a stock on its exchange. AAPL is Apple Inc. on the Nasdaq; MSFT is Microsoft. The convention dates back to stock-ticker machines that printed trades on paper tape, where a short, unambiguous code mattered for speed.
The same company can trade under different symbols on different exchanges — an international company often has a primary listing in its home market and a secondary one elsewhere, sometimes under a different code entirely. Exchange-suffixed symbols like VOD.L (Vodafone on the London Stock Exchange) or 7203.T (Toyota on the Tokyo Stock Exchange) disambiguate which specific listing a symbol refers to.
Tickers can also be recycled: when a company delists — through acquisition, bankruptcy, or otherwise — its old symbol can eventually be reassigned to an unrelated company. Always confirm you're looking at the company you intend to, especially for a symbol you haven't checked recently.