It means there are not enough players joining the game for it to start. Many game modes are not very popular, the best bets are turf takeover and mixed mode. Player count can vary a lot by platform and server location as well. What are your particulars in those terms? PC in particular can be lonely depending on what, where, and when you are playing.
A common issue is that it will not find a game and put you in a game by yourself even though there are games going on. I get this from time to time and usually going back to the backyard and jumping back in the portal will immediately find a game. I get this with the Mystery Portal a lot too.
On PC Turf Takeover is almost always available but the other modes not so much. I get my quests for the other modes done via private games with bots. If you're on console I'm not sure about it but TT has always been popular although I preferred mixed mode in GW1 so I could play all the modes, it seems less popular these days in GW2.
