What are alleles
Our genes are located in the cell nucleus, where they are packed into chromosomes. Chromosomes can be divided into conditional sections, like on a map, these sections are called loci.
In our organism, chromosomes are represented in pairs — one from the mother, the other from the father. During fertilization, they can exchange genetic material, resulting in a unique organism.
Since chromosomes are paired (they are called homologous), genes are also represented in two variants. This is called alleles, i.e., different forms of the same gene located in identical loci.
These genes can be identical or different. In this case, the organism is called homozygous and heterozygous respectively.
Let's take the eye color gene. There might be an allele for brown eyes, and along with it — one for blue eyes. In this case, the person's eye color will not be random. The thing is, there are dominant and recessive alleles. The brown eye gene is dominant, it will essentially suppress the other allele, and as a result the child will have brown eyes.
If both alleles are the same, then there's not much to choose from and the eye color will be blue.