Accounts are a list of transaction records by account - think of them like entries in your checkbook and savings account records. In this case you can use accounts to track donations of money, in-kind items (baby items donated, books given) or even things like patients visited or meals delivered.
Click on the Add account record link to add new values, right-click on existing ones to delete or edit them. The Summary link gives you a lifetime overview of this person's accounts history. You can right-click on any account record to send a letter to the person if you have any letter templates set up which are based on account records.