It means instead of dying out completely (like neanderthals, Peking man, Java man, etc) and being replaced by people migrating out of Africa around 100,000 years ago, they lived on, breeding with these Africans, creating a new race of men that still exist today in the various countries that make up the world.
It's as if a rather large family of cousins, brothers, etc lived on the frontier of America, say 200 years ago, and, by circumstance and what not, due to this and that, didn't have children, or they died, etc, so by now the last old man, the only descendant of this vigorous family, just died, taking the bloodline and family name with him forever. The area, however, was settled by people from the East so there's still people there, just not our original family.
This guy shows how the family didn't die out, because some of them married into the newcomers, and these people's descendants are still among the local inhabitants.
I believe it's a compelling argument. The out of Africa theory never explained the differences that are the various races of mankind, but if there were several migrations over the millenium, then this would account for these differences very nicely.