It means that the worker class (miners) doesn't start at block #1, and that some people have a lot of the coin already that might be used for your benefit (marketing the coin) or simply dumped in the market as soon ad the coin hit an exchange. The latter will be a very unpleasant surprise if you have been working hard to mine the coin or even spent real life $ on getting the coin.
A separate but similar issue i observed with the catcoin release: i mined for a few hours during the chaotic beginning, and got a thousand catcoins. I earned these legitimately, but put in absolutely no work to do it.
the same hash power today would get me 4 catcoins. this is a separate issue to premining, but i see a handful of people zergrushing a coin release also making out like bandits.
i'm not saying it happened with catcoin, but a dev could easily claim a coin is 'no premine', and quietly release the source for a few individuals to setup stable pools ahead of time. as soon as the coin is released, all of the dev's allies would turn the mining infrastructure on, and mine that easy 1-2%. Everyone else that follows wastes their resources on promoting the coin. the organised developer benefits big: huge windfall, propelled forward on the backs of the proletariat, and all with no premining.