I think it's something to do with the amount of natural concentrated ingredients, though since it's a bit of a nonesense word with regard to frags, there's not really a set rule. It "should" be more concetrated than an EDT, though.
It's also quite pretentious and seems to be used as a justification for Creed charging so much. FWIW, even the millésimes I have tried from Creed don't seem to be any stronger than an EDT. And they don't always have much correlation with longevity. Erolfa lasts an hour on me.
The traditional use of the word millesime has to do with the production year.
But CREED uses it as a designator meaning that the fragrance is of especially high quality, rich, and full. I think this is basically a marketing technique to differentiate their more recent offerings from those commonly referred to as the EDT's. If in fact there is any difference in the concentration of these two supposed groupings, it is pretty much negligible as all of their offerings are relatively rich fragrances.
What the others said. Think of Creed's Millesime line as eau de parfum strength. Whether or not that makes them any better, or stronger, than the eau de toilette predecessors is debatable. Regardless, the Millesime line is VERY concentrated, and a little goes quite a long way. So that little bottle you just dropped a cool couple of Benjamins on will last you a long while. Contrast that with Santa Maria Novella's eaux de cologne, which are weaker even than eaux de toilette. I practically have to bathe in SMN's Imperial Lavender to get any sort of staying power at all, making the $110 3.3 ounce bottle of it a much poorer value than a 2.5 ounce bottle of Creed's Green Irish Tweed that sells for almost twice the price. Having said that, I'd happily trade all my Creeds for the SMN lavender in terms of the scent quality.