Operating expenses (OPEX) are the costs of running a business on a daily basis. These include rent, utilities, salaries, insurance, office supplies, and marketing — basically everything except the direct cost of making your product.
For example, a coffee shop's operating expenses include rent for the shop, barista wages, electricity, and advertising. The cost of coffee beans and milk would be COGS (cost of goods sold), not operating expenses.
Keeping operating expenses under control is essential for profitability. Revenue minus COGS gives you gross profit, and gross profit minus operating expenses gives you operating income — the real measure of how well the core business is performing.