A Hip/Form curve is a large French curve that aids in pattern making by providing an elogated, curved edge for smoothing and correcting pattern lines.
What is it used for in practical terms? An example would help.
French curves, dressmaker curves or Hip/Form curves are useful when altering patterns (or drafting your own). You may need to smooth out the underarm curve when altering for bust size. You may need to adjust a side seam for a high hip on a skirt or pair of pants. You can also use it to measure around a curve to adjust princess seam pieces on a pattern. You may be able to find some videos with tutorials on how to use on on youtube or maybe the fabulous stitch people will make one demonstrating with their download?
I've used it to make the curve at the hip in skirts when making my own patterns. I also used it to make the bottom hem of an a-line skirt since it isn't straight all the way around. I've used it anywhere you need a nice curved line when making my own patterns.