This is the "Register" or the "Ledger" window. Transactions are entered here.
To enter a new transaction, just type into the last blank transaction. As well as just typing them in, you can use the Date Accelerator keys to quickly enter dates. Use the mouse or the Register Keyboard Shortcutsto move between fields, and press the Enter key or click record on the toolbar to record the transaction:
Notice how a the transaction has been recorded and a new, blank transaction has been created.
You can edit existing transactions in a similar manner. GnuCash will prompt you to confirm any changes you make.
After you have entered a few transactions, GnuCash will try to guess some or all of the remaining details of a transaction when you type into the Description field. If GnuCash has guessed wrong, simply type over or delete the incorrect information and replace it with the correct information.