In genetics, when alleles are codominant, they are both expressed in the organism and contribute to the phenotype.
The trait of human blood type can be used as an example. Blood type has three alleles--A, B and O. Alleles A and B are codominant while the allele for O is recessive.
In an individual who is AA, or AO, the phenotype will be type A. That is because A is dominant to O in the heterozygote. The individual who is BB or BO will have the phenotype B because B is dominant to O in the heterozygote.
However, an individual with alleles A and B will have the phenotype known as type AB and both genes are expressed in the individual. Their blood will contain antigens for A and B on the red blood cells. Because allele A and B are codominant, their gene products will be expressed in the individual at the same time.
The only way the recessive allele O is expressed is in an individual who is homozygous-- with two O genes.
Another example of codominance is seen in cattle---the allele for red coat color is codominant to the allele for white coat color. If an offspring has one of each allele--the color of the coat is called roan containing both red and white hairs.
In summary, when alleles are codominant, if an organism inherits one copy of each, both will be expressed in the phenotype.