From LeakyPedia

Blood purity is when wizards only have children with other wizards, and therefore their child has magic on both sides of the family. Some wizards are very fanatic about blood purity, and look down upon wizards that have a Muggle parent or parents. The Malfoy family is a good example of this, as Draco often mocks Hermione's parentage and calls her a "Mudblood".

Also see Major Themes