Repeat of what ByranC already said, but I had already typed it, so here is my response.
Album artist is basically a field used to unite tracks in a compilation album into one album. Each track can have a separate artist in the Artist field in a compilation album, but if Album Artist is set to the performing artist, then only those tracks with common Album Artist will be treated as albums. If all the tracks have a common Album Artist then MC will make them into one album. See Album Artist and Album Artist (Auto) in the Wiki.
Example - say you have a compilation album with tracks from 3 artists, A, B and C. You set Artist to A or B or C. But for MC to treat all the tracks as one Album the tracks need to have a common Album Artist. That Could be A, B, C or just the most common artist, say A. Or, you can make up a common name to use in Album Artist, like 70s Rockers. Or, people sometimes use Various or Various Artists for Album Artist.
See the Wiki article, which also explains Album Artist (Auto).