Simple Definition:

Commerce is the exchange [giving something in return for something else] of goods [things that can be bought or sold, like food or clothes] and services [tasks that people do for others, like fixing a car or cleaning a house] for money or other goods and services. For example, when you go to a store and buy groceries with your money, that is a form of commerce. Another example is a company that washes people’s cars in exchange for money.

Very Simple Definition:

Commerce is when people trade things with each other. For example, when you give a friend a piece of candy in exchange for a pencil, that is commerce.