What does it mean when it says 17 jewels in a watch?

It just means there's 17 jewels in the movement. Typically jewels are mostly used in mechanical watches. The jewels are tiny pieces of [usually manmade] rubies that serve as low-friction bearings in critical parts that move very quickly and very often. Using traditional materials may not work as the high friction may cause those parts to wear down more quickly, thus requiring more frequent (and costly) repair and maintenance services.