Final answer:
Money acts as a medium of exchange when used for transactions, such as paying for a meal at a restaurant. This role is distinct from its function as a store of value or a unit of account.
Step-by-step explanation:
When a restaurant accepts your $20 bill as payment for your meal, money is acting as a medium of exchange. This is the most important function of money, allowing you to buy things without needing to barter goods or services. As a medium of exchange, money must be widely accepted as a method of payment in the markets for goods, labor, and financial capital. This differs from money's roles as a store of value, which refers to how well it preserves its value over time for future use, and as a unit of account, which enables a uniform measurement of the market value of goods and services.