Something that exhibits purity hasn't been tainted by something morally or physically. Children, fresh snow, and solid gold are all known for their purity.
Purity comes from the Latin purus meaning "clean, clear, unmixed, chaste." When you're talking about something that is clean and unmixed, you are talking about something characterized by purity. This can refer to someone's character or intentions. It can also describe an object or element. When you're high in the mountains, far from the city's congestion, the purity of the air is a relief to the lungs.