Amy- It's a misnomer to call it the back loop... it's the same "loop" as you always knit- you are knitting through the back leg of the stitch... Usually you knit the leg on your side of the needle- but "TBL" you knit the half on the far side of the needle- this twists the stitch 1/2 twist... so instead of a \ / shape- the stitch you worked will be more like an X.
If that didn't make it clear as mud- try knittinghelp.com
love Sarah