it meens 'through the back loop'
a video can be found on the glossary page of this site if you scroll down to Ktbl (knit through the back loop)http://www.knittinghelp.com/videos/knitting-glossary
It is a way of doing a twisted knit stitch. Normally when you knit a stitch you put the needle in the leg of the stitch which is closer to you. when knitting tbl, you insert the needle in the leg furthest away from you.