When talking about audio layers, or the layers in one's audio data, it is referring to the type of method that is used to encode audio. This information can also be found in the video file, for the encoding of audio data from the video.
There are different kinds of audio layers. They are usually referred to as Layer III, II, and I. The most common definition of these encoding methods does not include Roman numerals. Instead, they are described as MP1, MP2, and MP3 turn.