Product virality is the ability of a product to spread quickly through word-of-mouth marketing. It can be measured by looking at the k-factor, which indicates how many invitations are sent out and accepted for any given product. A higher k-factor means that more users are talking about and sharing your product with their friends, leading to rapid growth and increased revenues. By understanding this metric, you can gain insights into user acquisition, retention, and engagement - all key components for successful business growth.