Here's a breakdown:

* Gene flow is the transfer of genetic material from one population to another.

* Alleles are alternative forms of a gene. For example, the gene for eye color has different alleles for blue, brown, green, etc.

So, when individuals migrate from one population to another, they carry their alleles with them. This introduces new alleles into the recipient population, changing its gene pool and potentially altering the allele frequencies.

Let me know if you'd like more detail on the impact of gene flow!