The statement showing revenue minus costs for a period. Are you making money or losing it?
The P&L (also called 'income statement') is one of the three basic financial statements. It shows: revenue, minus cost of goods, equals gross profit; minus operating expenses, equals operating profit; minus taxes and interest, equals net profit. Monthly and annual P&Ls are how you know whether the business is healthy.
If you run a business, you should be able to read your own P&L without help. Most owners can't, which is why they make bad decisions. Spend the time to understand every line. The story of your business is in the P&L.