Immigration is when members of a species move into a community.
Emigration is when members of a species move out of a community.
Immigration and emigration can affect a community. It will affect the overall population of the community. It can also have an effect on the food web of the community. For example, if a certain food source becomes scarce to a species of birds in a population some of those birds might emigrate out of the community. Because there are fewer birds it will have an effect on the animals that prey on those birds.