A revenue expenditure is a cost that is expensed in the accounting year in which it is incurred. In other words, the cost will be matched with the revenues of the accounting year in which the expenditure took place. (This is in contrast to a capital expenditure in which the cost is deferred to the balance sheet and is then expensed over several accounting years.)
Revenue expenditures are often discussed with costs spent on fixed assets after they have been placed in service. For example, the amount spent each year to keep an ice cream's store's equipment working efficiently is a revenue expenditure. Also, the cost to repair the equipment will be a revenue expenditure. In both of these situations, the amounts spent will be debited to Repairs and Maintenance Expense and will be matched with the revenues on the current year's income statement.
On the other hand, if the ice cream store incurs a large cost to improve the equipment (to make it more than it had been) and/or to extend the equipment's useful life, the amount spent is considered to be a capital expenditure. As such, the amount is initially deferred to the balance sheet (capitalized) and will be expensed over the current and future years of the equipment's useful life.