So, imagine we live in a fairly remote village in the mountains. We have commerce with a dozen other villages, and it is the custom for women from one village to marry men from another. This intermarriage is a good thing, as it mingles alleles in what could be several isolated gene pools, like little streams flowing between ponds. This is gene flow. It maintains a greater allelic variety by exchanging traits between populations, and, in theory, prevents speciation.
One hard winter, there is a terrible avalanche, and the village at the edge of this community is permanently cut off from the other villages. During this hard winter, half the children died of starvation. A band of young men went off to steal brides from the wild nomad tribes far to the east, but never returned. There were only twenty people of procreating age left in the village, and several of them were somewhat related to me. We renamed the village Mondoshawania, since we were almost the founding family of the new composition of the village. The prevalence of some alleles for recessive traits was drastically increased (like my cousins' red hair) or decreased , as the genetic makeup of this one family was overly represented now in the gene pool of the village. This is the founder effect, and it is seen in bottleneck events.