Operating expenses are the ongoing costs of keeping a rental running: property management, insurance, property taxes, routine maintenance and repairs, utilities you pay, and similar recurring items. By convention they exclude your mortgage (debt service), depreciation, and capital improvements, which are treated separately.
Operating expenses are what you subtract from collected rent to reach net operating income, so how you classify a cost matters. A repair is an operating expense deducted now; a major improvement is capital spending recovered over time. Tracking operating expenses by category all year is what makes both your NOI and your tax return come straight from the books.