HSS is a type of tool steel, as opposed to carbide or other metals cutters are made from. HSS and carbide will both work.
HSS supercedes high carbon cutters, there’s some interesting history there from 1940s machining technology.
As far as types, this kit (which you don’t need) has a nice selection of shapes and sizes you can reference off of: https://www.amazon.com/Freud-Router-General-Purpose-87-208/dp/B01LX903DH
It’s the kit I use, but you definitely don’t need to spend that much. A variety of straight and V cutters, some tapered ‘pointy’ bits for fine detail, whatever you need really.
Edit: Thanks SDJ, I also use Whiteside bits on my router, they are very high quality. Also Rockler makes (relatively) inexpensive good quality router bits, likely would work for the SM also. These guys.