M4A stands for MPEG 4 Audio, and it is a popular file extension used to represent audio files. Most people are familiar with MP3 and how it shrinks down the file size of songs and other audio files. M4A and MP4 do the same thing as MP3 does, but even better. Quality is better and file sizes are usually smaller than MP3 files.
Nero burns M4A just like MP3's.