TL;DR: A diploid organism with two alleles of the same gene (heterozygous genotype) may have a phenotype expressing either white or red pedals (dominant-recessive inheritance), a mix of both (intermediate inheritance), or a combination of both alleles (codominance).
Let's assume a diploid organism, i.e. one with two sets of chromosomes, such as a plant with a beautiful flower. This plant has two different alleles of the same gene, each expressing a different flower color. In other words, the plant has a heterozygous genotype.
Now, let's explore what the phenotype of this plant would be.
In the case of dominant-recessive inheritance, only the dominant allele is expressed in the phenotype. This plant would have either white pedals or red ones.
In the case of intermediate inheritance, we would have a mix between both. A certain protein is expressed that is influenced by both alleles. We would end up with a pinkish flower.
When we consider codominance, both alleles are expressed independently. Thus, we would find either white or red pedals, as opposed to the intermediate case, where both alleles contribute to one protein.