A one step checkout is a simple checkout process where the customer who was already registered before only need to provide his/her email address and your store will take him directly to the FINAL checkout page where the [place order] button is located.
So he does not have to provide his billing/shipping address again and not even his credit card information.
Of course you will need to find a payment gateway that can save the credit card info for you, so you can remain PCI compliant.
Also, you will need a shopping cart that make use of such payment gateway and can offer such 1-step checkout.
(paypal quick checkout is an example of a 1 step ckeckout)