It is December 31. Your year had ₦100,000 of revenue and ₦40,000 of expenses, leaving ₦60,000 of profit. To start the new year clean, you 'close the books': revenue and expense accounts (the P&L) get zeroed out, and the net (profit) rolls into Retained Earnings (equity). Balance sheet accounts (cash, wallets, etc.) carry forward untouched. This is the rhythm of accounting that engineers building reporting systems must understand.