Call signs, or call letters, are how radio (and television) transmitters are officially identified, including broadcasters and other users of the electromagnetic spectrum such as ham radio operators and military transmitters. There is an international system to allocate ranges of letters and numbers used as prefixes to call signs. These assignments can seem fairly chaotic due to their original allocation in an international conference in 1912 when geopolitics and technology were very different.
In some places, radio and TV stations are commonly known by their call signs, for instance in the United States where such signs (for broadcasters) are usually 4 letters (occasionally only 3) starting with W or K (usually W east of the Mississipi and K west of it, but there are some exceptions), but there are other countries that rarely identify broadcasters by call sign, using other naming systems or omitting the international prefix when identifying stations domestically (e.g., in Australia the prefix is rarely given). Even in countries where call signs are used, there are often "marketing names" for stations such as "K-104" or "Lite 98" that are better known to listeners than the actual call letters, though national regulations may still require the station to announce the call letters at some interval (e.g., hourly).
Other sorts of transmitters such as ham radio operators also have call signs, with the same set of prefix letters, but possibly different lengths from commercial broadcasters. Some call signs have numbers in them in addition to letters; for U.S. ham call signs, the number denotes which region of the country the operator resided in at the time of licensing (though they may move around; the call sign attaches to a particular individual rather than a fixed station).