At the end of every year (or quarter or maybe even monthly) after you complete your auditing, you close your books of accounts. This means that you make all your special entries like:
etc. and book your Profit or Loss.
By doing this, your Income and Expense Accounts become zero and you start a new Fiscal Year (or period) with a balanced Balance Sheet and fresh Profit and Loss.
In ERPNext after making all the special entries via Journal Voucher, you can make all your Income and Expense accounts to zero via:
Accounts > Tools > Period Closing Voucher
The Period Closing Voucher will make accounting entries (GL Entry) making all your Income and Expense Accounts zero and transferring the balance to the Account you define.