Mastering is the process of moving audio from recording device to the storage device. All copies after that are made from the master copy. Master recordings (usually called just "masters") can be made on discs, tapes, and computer data storage formats.
A multitrack master is an original multitrack recording, which may be worked on over time. This may have four, eight, sixteen or more tracks. This kind of recording cannot be played on an ordinary machine, and needs a special machine to be played or recorded.
Multitrack recordings are mixed when they are finished, into a mono (monaural), stereo, or Surround Sound recording. This is called a mixed master. Copies of the mixed master can be played on an ordinary machine.
Master recordings are often valuable, especially if the recording artist is popular. Most masters are owned by record companies, but many artists and bands own their own master recordings, or are able to buy them later.