They're referring to engine displacement. 400cc and 600cc. You'll find manufacturers tend to make roughly the same engine sizes for different bike types. Like 150cc, 250-300cc, 500cc, 600-700cc, then 1000-1300cc. Since you're starting out, a good idea is to start small like somewhere between a 300-500cc bike. Typically, the larger the displacement, the more power the engine can produce. Practice your skills and get a good foundation for actually controlling the bike at slow and high speeds. A Honda Rebell 500 (cruiser) can go 90mph no problem and is light, nimble, relatively easy to work on, and easy to control at slow speeds since it has a low center of gravity, so it's a bike you can start with and keep for a long time. Honda uses the same 500cc engine in different styles of bikes but with different tuning, like the cbr500 (sport), CB500 (standard), NX500 (dual sport). Eventually if you decide you actually like motorcycles and want to "upgrade," you can get a bigger displacement bike somewhere between 600-1000cc, depending on your needs/interest. What kind of style do you like? Cruiser, sport, standard, etc. You can't go wrong with Honda, Kawasaki, or Yamaha to start off with. They tend to have decent maintenance intervals and are easy to work on.
Like a Kawasaki versys 1000 kind of sport tourer or more like a GSX-S1000GT? I've got a versys and it's great.