Polygenic refers to DNA traits such as skin color, hair color, eye color, and stature that are influenced by multiple genes rather than other traits that exist as a yes or no (such as gender or blood type). The color of skin, hair, and eyes are the result of the combination of multiple genes which is why such physical traits can vary drastically between siblings. This is because each parent contributes one of their chromosomes to the offspring.
For example, individuals with darker complexions and hair will contribute skin and hair genes that may or may not mask lighter skin or hair genes contributed by a partner. This makes possible several potential combinations of genes that will result in several possible results including offspring having a darker complexion, a lighter complexion, or a mixture of the two.