It's a way to increase one stitch. You knit the stitch as normal BUT DON'T SLIP OFF THE LEFT NEEDLE, then you knit into the back of that SAME stitch again and slip it off. You will now have 2 stitches on the right needle.
Amy has a video of this in the increases and decreases section in basic techniques. It's my favorite way to increase.
Good Luck and HAPPY KNITTING :XX: :XX: