The k2tog (knit 2 together) and k2tog TBL (knit 2 together through the back loop) are both decreases that will remove one stitch from your knitting. The main difference between the two is in the orientation of the stitches: k2tog crosses the stitches from the front, while k2tog TBL crosses them from the back.
This has a few consequences. First, k2tog TBL is a left-leaning decrease, while k2tog is a right-leaning decrease. This can be important when you're working on patterns with multiple decreases - you'll want to make sure that all of your decreases are leaning in the same direction so that your finished piece looks neat and symmetrical.
Second, because of the way it crosses the stitches, k2tog TBL tends to be a tighter decrease than k2tog. So if you're looking for a very subtle decrease, or one that won't change your gauge too much, stick with k2tog. But if you don't mind your stitches being pulled in slightly and want a more pronounced decrease, go with k2tog TBL.