knit 2 stitches together through the back loop
A K2tog tbl simply means knit 2 together through the back loop.
This is a left-leaning decrease stitch.
I’ll walk you through this one. With your right needle, insert it into the back of the next 2 stitches on your left needle, at the same time, like this. Now wrap your yarn around your right needle like you would for a normal knit stitch, bring up your new stitch and slip off the 2 stitches from your left needle.