Gene flow is the
geographic movement of
genes that results when individuals move from one area to another. For example,
animals that leave the population they were born into and go join a different breeding population are one vector for gene flow. Plants are stuck where they are growing, but their
seeds and
pollen can get carried far away from their original home, resulting in a flow of genes into a different population.
Evolutionarily, what are the consequences of gene flow into a population? It's possible that it might destabilize a population, and keep it from becoming as well-adapted to its local environment as it could be, as the genes that are coming in are likely better-suited for their environment of origin. However, it's also possible that introducing novel genes into a population provides more variety for natural selection to work on, thus rendering the population better able to adapt.
Gene flow between populations of organisms belonging to the same species is often credited with preventing divergence -- that is, preventing different populations from evolving in their own way to the point that they become different species.
It is also possible to have gene flow between species which are not completely reproductively isolated, and where hybrid individuals are capable of reproducing.