Real Stuff, Tradeable
Three scenes from the same global market
A bulk carrier loaded with iron ore leaves Tubarão, Brazil, headed for a steel mill in Qingdao, China. The cargo is worth tens of millions of dollars, and nobody on board cares which exact rocks are in the hold. As long as the iron content matches the contract, one shipload is as good as another.
A copper mine in Chile pours ingots stamped with a grade. They'll travel to a warehouse in Rotterdam and sit there until someone buys them. The buyer doesn't care which ingot, just the grade and weight.
A wheat farmer in Kansas locks in a price for her July harvest in February. The buyer she'll deliver to in July hasn't even been picked yet. She's selling to a market, not a person.
These three things have something in common. They're raw, physical, and broadly fungible (one barrel of Brent oil is interchangeable with another barrel of Brent oil at the same grade). That's what we mean by commodities. Stuff comes out of the ground or grows in a field, gets graded, and trades on global markets.