Dominant and recessive allele
When an allele is dominant, one copy of the gene is enough to produce the respective phenotype. In order to exhibit a recessive phenotype however, an individual must have two copies of the gene.
Dominant alleles are usually denoted by capital letters, recessive alleles by lowercase letters. In the case of eye colors for example, "brown" is dominant over "blue". Obviously individuals with two alleles for the color brown (AA), have brown eyes. Individuals with one allele for the color brown and one for the color blue (Aa) also have brown eyes, because the allele for brown eye color is dominant. An individual with blue eyes possesses two (recessive) alleles for the color blue (aa).
Figure 1: Genotype and corresponding phenotype for eye color.