I am no economist but in simple terms, an economic good is anything that has some value or "utility" that you can buy or sell in the market.
The computer or other device that you are reading this on, the internet connection you are using, the food that you eat, the clothes that you are wearing - all are economic goods. You buy them, and you get something (utility) out of it.
But air is not an economic good because you are not paying for it, even though without air you cannot live. The reason why air is not an economic good is it is available everywhere, or in other words, it's not scarce. So you cannot sell air because nobody wants to buy.
Our world is full of things that are scarce and provide utilities to us and people are willing to pay for them. Hence ours is a world of scarcity, full of economic goods. Does it make sense?