Humans have several eye color genes. Genes come in flavors called alleles. The bey2 gene has two flavors - brown and blue. The gey gene also has two flavors - green and blue. Genes are on chromosomes.
Chromosomes come in pairs.
Thus each individual has two copies of each gene. Genes are used to produce proteins. A gene that comes in two flavors might come in one flavor where the protein works correctly and another flavor in which the protein does not work. Thus an individual with one copy of the good flavor and one copy of the defective flavor for a gene could still produce a protein that worked. This is the simplest case of dominance.