PU loop from BO and k2tog is used as part of making a particular buttonhole. This is one of the many, many ways or situations to pick up a loop. I differentiate picking up a loop from picking up a stitch when there is no actual stitch being picked up (and cast ons and bind offs aren’t really stitches).
PU is the abbreviation used for pick up.
BO is the abbreviation used for bind off.
k2tog is the abbreviation used for knit 2 together.
In this case, the PU loop from BO is worked as part of a knit 2 together to close a gap created by the bind off done on the previous row. By knitting the loop together with the next stitch on your left needle, the gap is closed and the stitch count doesn’t change.
PU loop from BO and k2tog
Use the right needle to pick up a loop of the bind off done on the previous row and put it on the left needle. Knit the loop and the next stitch together as a k2tog. The stitch count doesn’t change and the gap created by the bind off is closed.