Before a Profit & Loss statement or a Trial Balance can be generated, you have to post entries to the General Ledger. Creating customer statements, Payroll balances, cashbook (bank) reconciliations and several other reports also require posting to the General Ledger.
The General Ledger is used to create balances for balance sheet accounts and totals for profit & loss accounts.
Posting will also ensure that the latest journal entries are posted to the GL.
To post, go to the Ledgers screen, click the General Ledger Tab and select the option directly below the Tab list. There are several links to "Post to GL" on different screen where its regularly needed.
Always remember to ensure that your Financial Year is current before posting to the GL.