So they're flurocarbon strings, which is a fancy way of saying fishing line, but then again most ukes come with nylon strings so these aren't exactly exotic materials.  If you're not used to this style of string it's worth trying, and this is probably a good brand to try since they're good quality and affordable.  Personally I like the feel of a flurocarbon string so after switching my ukes to some of the more expensive strings I decided to switch a laminate uke or two over as well and the Martins are just fine for this purpose.