To knit into the front and back of a stitch, first knit to where you'd like to make the increase, or where it is indicated in the pattern,
then insert your needle into the stitch as normal,
and knit the stitch,
Don't push the stitch off your needle, but instead reach behind with your needle and insert it into the back of the stitch,
Wrap the yarn around the bottom needle (right hand needle) and knit that stitch too.
Now you can push that stitch off...
In the picture below, you can see the two stitches made from one stitch...note the little "bar" it creates.
In this picture I've knit a few rows so you can see what the stitch looks like...see the little horizontal bar?
There you have it...the KFB stitch...happy knitting to you dear stitchers!