Can someone tell me what this means and how to do it? Thanks in advance!
Yarn forward just means to bring your yarn to the front as if you are going to do a purl stitch. This link has a photo of what the yarn forward looks like – I think its Step 2.
Yarn forward (yf) can also mean a yarn over when you bring the yarn to the front and then wrap it around the right needle.
This is an increase and is used in lace or eyelet patterns.