Using flowers as an example, a red flower and a white flower are being mixed.
If incomplete dominance was to occur, the offspring flower would be pink because the red and white mix.
Another example of incomplete dominance is a woman with curly hair and a man with straight hair having a child with wavy hair. Neither of the genes are full on dominant in the offspring.
Codominance on the other hand, for the flower example would have a flower where some petals and red and some petals are white. Both characteristics of the gene are shown in codominance.
Hope this helps!