In every human, plant and animal, there are two different alleles, one from the father and one from the mother. One allele is dominant, the other is recessive. That’s why you have your mother’s hair instead of your father’s, for example.
But there are exceptions. If you cross a red snapdragon with a white one, for example, you will get pink flowers. That is because both of the alleles are dominant. They combine. This is called Incomplete Dominance.
There is also another kind of dominance. It’s called Codominance. It’s when (let’s stick with flowers) a camellia, like the one below, is two colors. Both alleles are dominant, but instead of combining, they ‘share’ the flower.
It’s not so simple, is it? Basic dominance is only the beginning. Who knows what might be discovered in the future?