what is this and how do I do it? I want to put a tiger paw on an already knit hat

Essentially, it's just using a darning needle to embroider over your knit stitches. There is a good tutorial here:

http://www.purlbee.com/duplicate-stitch/

Incidentally, you can also use duplicate stitch for weaving in ends. There's an article about that here:

http://knitty.com/ISSUEfall04/FEATfall04TT.html