A SAP file is an audio file commonly used by Atari systems that have the POKEY sound chip. It contains a "chiptune," which specifies a sound format that allows the sound card chip to synthesize and sequence sounds in real-time. SAP files are typically used for playing back music from old Atari systems on a modern computer.
You most likely will only encounter an Atari SAP music file if you enjoy listening to retro music from Atari systems. SAP files may be created by Atari fans, but they are typically ripped from the Atari systems and uploaded to Atari fan sites for other enthusiasts to download.
SAP files can be played and converted to other formats by a variety of programs in Windows, macOS, and Linux. These applications emulate the Atari 8-bit computer architecture and POKEY sound chip in order to playback SAP files. Programs that can play SAP files include SAP Player, Audio Overload, Cog, PenguinSAP, and ASAP.