Kfb (knit front and back) is an increase stitch worked on the knit side of the fabric. You are basically knitting into the front and back loop of a single stitch to make two stitches. This stitch increases the total number of stitches on your needle by one.
How to work a Kfb increase
So I have cast on 10 stitches and worked a few rows in stocking stitch, on the next knit row the instructions are :
K4, Kfb, knit to end. (11 sts)
I have knit the first 4 stitches of the row and am now ready to work Kfb:
Step 1 – Work the next stitch as if to knit, but do not slip the stitch off the left hand needle.
Step 2 – Insert the tip of the right hand needle into the back loop of the same stitch on the left hand needle.
Step 3 – Pass the yarn around the right hand needle as you would to make a normal knit stitch.
Step 4 – Pull the yarn through the stitch, slipping the original stitch off the left hand needle.
One increase completed.