Dominance, in the simplest Mendelian terms, is when one allele of a gene masks the expression of the other gene found at the same place. For example, if M codes for brown fur (and is the dominant allele) and m codes for black fur (and is the recessive allele) the organism would need to have both of their alleles be m to express the phenotype of black fur. If either one of the alleles is M then the organism will express a phenotype of brown fur. This is of course a gross oversimplification and has many exceptions due to other things going on that may affect the outcome. Such as if the trait is inherited maternally for example.