Loss is an accounting concept, the exact opposite of profit, normally the bottom line of the income statement, which is also called profit or loss statement. Start with sales, subtract all costs of sales and all expenses, and that produces profit before tax. Subtract tax to get net profit. If the end result is negative, then instead of profit it is called loss.
Check out our article What Is an Income Statement? as well as our article on the LivePlan blog, How to Read and Understand Your Profit and Loss.