The metric k-factor is a measure of the virality of your product or service. It is calculated by dividing the number of invitations sent out for your product by the number of new users acquired through those same invitations. This metric is used to gauge how quickly your user base is growing and measure its overall performance in terms of word-of-mouth marketing. A higher k-factor means that more users are inviting their friends to use the product, which can lead to rapid growth and increased revenues.