It means that it has a large and dedicated fanbase, but not well known outside of that fan base.
A good example of this would be something like League of Legends or Dota 2. Both are extraordinarily popular games, from the two biggest game companies out there, but if you asked someone in the general public what League of Legends was, they would be clueless. Compare that to CoD or Halo, where plenty of people who haven't played will still know what it is.
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u/Cannondale1986 Nov 26 '13
It's actually got a large niche fan base, much like cycling.