M4A is the audio part of an MP4 video container. M4A can also be saved as an individual .m4a file and be played outside of being in a MP4 video. Nowadays most modern devices that can play music by default supports m4a file format out of the box.
More traditional audio players like a music player in an older model car, an old MP3 audio player, or a music player such as SONY Walkman may not support M4A as it’s a much newer format in relative codec lifespan.
M4A typically uses AAC codec to compress its audio data, this is much more efficient than more widely known formats like MP3, OGG etc.