Credits addon brings a deposit system to your website where your clients pay to buy “credits” (in some e-commerce systems also known as “Wallet”). Then they can partially or fully use it to book services. Admin can also manually add credits to any client (WordPress user).
Selling credits example is given on this page (You must login to use Credits). Using credits is similar to paying with any other payment gateway, e.g. PayPal, Stripe, and you can find an example in Payment Gateways page.