To prepare a trial balance, you or the accounting software will simply list the titles of all of the accounts in the general ledger that have balances. To the right of the account titles you will have two columns for entering each account's balance. One column is headed "Debit" and the other column is headed "Credit." You then list each account's balance in the appropriate column. After all of the account balances are entered, each column is summed. The total of the debit column should be equal to the total of the credit column.
Prior to accounting software, there were many opportunities for errors. For example, an amount might be written incorrectly when posted from a journal to the account, a math error might occur when calculating an account's balance, an amount might be written incorrectly in one of the columns on the trial balance, and so on. The trial balance alerted you that an amount or amounts were wrong. If the trial balance did not balance, it meant rechecking all of the amounts.
Today, the accounting software has eliminated the math and clerical errors. As a result, the trial balance does not play the critical role that it did many years ago.