Mix tapes were a popular way for friends in the 80s and 90s to share favourite songs with one another.
Many people owned CD or radios which incorporated a cassette player, which enabled recording from the radio or CDs. Double tape decks could enable recordings from cassettes too.
Cassettes tapes with blank inserts were purchased, so the maker of the mix tape could create a play list and particular messages, as well as naming the compilation, and/or who it was addressed to.