A Patternmaster is a "ruler" with a number of different markings and curves that can help when pattern drafting. To me though, it is a bit of a luxury item. I struggled to decide whether it should actually be on my list of “pattern cutting tools I cannot live without,” but it just managed to scrape in.
I know a lot of people swear by them, but I often find it much easier to draw a curved line by pivoting with a straight ruler. The main thing I use my Patternmaster for is drawing right angles – it is very good for that, and marking the bias, if required.
It can be used for drawing tight curves (like armholes) or looser curves (like the hip curve). It can also be used to add seam allowance.