The matching principle refers to matching related income and expense items in the same accounting period. For example, if you had a store and had a big sale event on the last day of the month, and recorded all the income for that day, you would also want to make sure you accrued all the expenses related to the event (advertising, etc.) even though some of those expenses might not be paid until the following month. Similarly, if you had paid any expenses in the month before the sale, you would want to defer those expenses and call them "prepaid expenses" until the month of the sale.