It refers to the gradual and directional process of species change in a community. Eventually, succession leads to a climax community.
Climax community: A community that remains fairly constant in species composition if the land and climate are undisturbed. These are the communities that "define" the various biomes.
Pioneer species: The first species to colonize an area... usually lichens and mosses, beginningthe process of soil formation, but sometimes higher plants.