Backing tracks are recordings that are designed for practicing musicians to play along with. Originally these were called 'minus one' recordings. They were 'minus' whatever part the musician would be practicing. So if you're looking to practice lead guitar, the recording will not include a lead guitar part. If you're looking to practice a bass line, the recording will not include a bass line etc...The idea is to create an experience similar to playing along with actual musicians. There are many of these tracks available on YouTube and they vary greatly in quality. I've posting some of the better ones I've found here. Have fun!