Clay credits are the currency you use to purchase different data points and actions in Clay. The vast majority of actions in Clay are either free, or cost 1-2 credits, but the average cost of a data point or action varies depending on the provider.
For example, finding an email costs 2 Clay credits if you use Prospeo, DropContact, Datagma, or Hunter.io. However, finding an email from People Data Labs costs 5 credits—simply because we pay more money to get the same data from People Data Labs vs. others.
Some data points, like mobile numbers, are harder to find and are more expensive as a result. Finding a mobile number starts at 2 credits, but it can cost 5 (People Data Labs), 13 (Selligence), 15 (ContactOut), or even 25 (Datagma) depending on the provider you use.