Genetic flow, also called gene migration, is the transfer of genes between different populations through migration. Immigration is when new organisms join a population, changing allele frequencies. Emigration is when members of a population leave, taking with them their genes. These change the overall balance of the gene pool of the populations these species join or leave. Gene transfer is the flow of alleles from one species to another. Gene transfer is especially common in bacteria.
In evolution one of the things that can cause genetic change within a population is a concept called gene flow. You may have heard of genetic drift that's when you have within a population you have random changes or gene flow is when you have your population of organisms and then either through migration. Immigration new people come in or new organisms come in bring with them new stuff or by emigration exiting you have some people leave and that can cause some changes in the relative frequencies or percentages of genes within that population so genetic flow again is the transfer of alleles from one population to another through migration. Immigration again it's coming in people bring in new alleles or they perhaps a larger group of people have a lot of a particular allele and they can change the overall balance of alleles within the gene pool of the population they just joined versus immigration when emigration sorry when you take a group subset of your population and remove them they take away their genetic nature perhaps lowering or altering their frequency of the region population.
Now sometimes kids get this kind of confused with the idea that's a little bit weird and that's called gene transfer and that's not when members of a different population but still the same species leave or join you, it's when genes are, alleles are transferred from one species to another. Now this is not that common for you to know about but actually happens a lot in nature. Some of the ways that this can happen is bacteria are notorious for this, they can take genes from other bacteria when some bacteria die other bacteria can take in their genes it's freakishly weird. Viruses are another way that you can have gene transfer because viruses can sometimes cross from one creature to the next and there is a lot viruses like HIV for example a lot of viruses can insert their DNA into the host cells DNA and they may actually be able to transfer gene from one host cell from say species A into the host cell of species B. Plants can also do this kind of gene transfer if sometimes if you have hybridization and this can happen in another kind of creatures besides simply plants but hybridization when you have two species cross sometimes you windup adding some genes from a new spec- from the old species unto the new species and so gene flow or genetic transfer that's when it's within the same species through migration gene transfer is between species.