A dominant allele is a variant form of gene that is expressed phenotypically in the presence another variant form of the same gene.
For example Both A and B alleles are dominant over the O allele form of the gene for red blood cells.
O is a recessive form of the allele at the gene site. It is likely that the O allele is the absence of any proteins being produced by the DNA at the gene site
A person whose genome is AO will have the phenotype of A. The A protein on the red blood cell is the only protein that will be expressed.
A dominant allele on the DNA like A will be expressed over other alleles at the same site on the DNA.