These are your basic settings for the payment system. The first three checkboxes refer to two systems in place to track payment information. If the first box is checked, a ledger is kept for each order showing payments received/refunded along with the current balance for each order. If the second box is checked, you may grant permission for people to delete payment lines from these ledgers in the event of mistakes. The third box makes sure payment information is copied to the order log which cannot be deleted.
Finally, at checkout, when a customer is choosing their payment method, Ubercart automatically loads up an extra details that must be filled out by or displayed to the customer. In the event that the payment method requires no further information, this text field lets you specify a default message that gets displayed to the customer so they know it is safe to proceed with checkout.
The table at the top lists the different payment methods available on your site and lets you choose payment gateways to process those types of payment. For the default check and other methods, no gateway is required. If you've installed modules that define extra payment methods like the Credit module, you may also want to install a payment gateway module that will let you process credit cards through your site. The select boxes here let you choose the default processor per payment method.
Payment methods require their own settings, some more than others. To properly configure the payment methods in use on your site, come click on the title below the main table to expand a group of settings for that method. Be sure to save your change when you've finished editing them, and be careful that you don't click the reset button accidentally!
There is a short description of what payment gateways are here. Different payment gateway modules may require you to fill out different settings before being used to process payments. When you install a new one, be sure to come back here and configure it properly.