The abbreviation yf stands for yarn forward, an instruction to bring the working yarn between the needles to the front of the work. It is used to transition from a knit stitch to a purl stitch, to slip a stitch with the yarn in front, or sometimes as a specific type of yarnover.
Heard on the job
The pattern says knit two, yf, slip one, yarn back, knit two.
Related terms
Wyif
The knitting instruction for with yarn in front, directing the maker to hold the working strand toward them.
YB
A knitting instruction directing the crafter to move the working yarn to the back of the work.
Wyib
The knitting instruction for with yarn in back, directing the maker to hold the working strand away from them.