Incomplete dominance is a phenomenon in which the alleles of two genes are not completely dominant over each other and so when they are present at the same time in an organism (heterozygous for that gene) the organism will have an intermediate phenotype that is a combination of both phenotypes.
For example: The flower color in Snapdragon can have three different genotypes as well as phenotypes. The flowers can be red, pink or white depending on their genotype. The allele for red flower (R) is incompletely dominant over allele of white flower (r). A plant with genotype RR will be red flowered and that with rr will be white flowered. However, if a plant has genotype Rr (intermediate), then the flowers will be white in which no allele was dominant over the other and both showed their effect giving a combination of two colors as Pink flowers.
Same is the case with the mating between white and black hen, where offspring was grey in color. This grey was a combination phenotype of two phenotypes.
