YFWD (yarn forward), YON (yarn over needle), YRN (yarn round needle) and YFRN (yarn forward round needle) are all variants of the plain old YO (yarn over).
The four terms YFWD, YON, YRN and YFRN are basically just letting you know which different stitches (knit or purl) are in front of and behind the yarn over.
YFWD (yarn forward): A YFWD is a YO between two knit stitches. After your knit stitch your yarn is behind the needles. Bring the yarn forward between the needles and then knit the next stitch.
YRN (yarn round needle): The YRN is a YO done between two purl stitches. Purl one stitch and you’ll find your yarn is in front; bring the yarn all the way around the needle until it is back in front again and then purl your next stitch.
YON (yarn over needle): The YON is a YO done after a purl and before a knit stitch. Purl one stitch and your yarn will be in front; bring it back so it is behind the needle and then knit the next stitch.
YFRN (yarn forward round needle): The YFRN is a YO done after a knit and before a purl stitch. After you knit your stitch, the yarn will be behind your needles. Bring the yarn forward and then all the way around the needle until it is in front again before purling the next stitch.