HIi lle7
"Tab" is a short form for tablature, which is a format (rather than standard notation) to show how to play a song, riff, etc. It has 6 strings like the guitar and tells you which finger to play one each string. For the most part, it does not tell you the timing like standard notation does. Here's a link that will show you how tab works.
Introduction to TABS There are lots of sites out there explaining how tab works and the tab for many songs.
So, if you don't know the key of the back track (some will give you the key and or chords) you can search for the tabs of the song and see if you can find one in the same key as the back track.
Hope this helps some. There are many here that are more informed than me, but this should get you started.
Good luck.
Nutty