What they mean is probably going to depend on the person. Some people mean it's good for someone new to penetration. Sadly, a lot of times people use it when the vibrations are lackluster, assuming that someone starting out won't need strong vibrations. Many people have commented that they've needed strong vibes right away, so it's misleading to say something is "good for a beginner" when you mean "weak."
I think people do this as a way of not being so negative about a product. Lots of people don't want to flat out say "it sucks!" so they try to find a positive spin. I guess saying "it's good for beginners" is a way to make a review sound less negative.
I also have issue with people who use "good for advanced users" to mean "large size" or "lots of texture." There are plenty of people who have used toys for years that prefer smaller girth or less texture. I've had sex toys for over 10 years and I can't stand textured things. I don't think that makes me less "advanced."
Generally in my reviews I'll state "good/bad for someone new to penetration" in the size section. So if something is 1.75" diameter, I try to caution that it's probably not the best toy to have your first penetration experience with. However, I'll also add it "good for someone who prefers smaller/larger toys" to encompass those who just have preferences either way independent of how long they've been using sex toys.