You use it to define accounts and transactions. Accounts are things like inventory journals, bank accounts, and employee records, while transactions are things like purchases and sales transactions, deposits and withdrawals, and time cards and paychecks.
Transactions are essentially time-stamped records, while accounts are a lot more complicated.
Each account keeps track of the transactions that effect it. It also has a set of attributes, which are basically running totals computed from the transactions. Once an Accounts system is set up, you use it by creating transactions and by querying the attributes of accounts. Queries can be from a simple GUI tool, from reports such as bank statements or monthly reports of sales, or from programs that are creating transactions, like a check-writer that uses a querie to decide how large a check to write. All normal interaction with an Accounts system by creating transactions and querying attributes.