When speaking about audio layers or the layers of an audio file, it refers to the kind of encoding technique used to encode an audio. This information also occurs in video file when it comes to the encoding of the audio data of the video.
There are different types of Audio Layers. They are typically known as Layer I, II and III. A more common notion of these encoding techniques does not contain Roman numerals. Rather, they are expressed as MP1, MP2 and MP3 respectively.