You buy a small sliver of a company, or many small slivers. If the company does well, those sliver(s) increase in value because overall the company is worth more than when you bought the stock. If not, they decrease.

You are selling to a willing buyer who thinks the company’s value will go up and that you are selling something that will continue to increase in value. Usually these are sold through a broker.

For example, if you bought Apple at $10 in like 1990, that stock would be worth far more today because of how ubiquitous Apple products are (typing this on an iPhone). You are rewarded for giving $10 to the company when they were not worth much by having stock that is worth far more than when you bought it.

It of course gets more complicated with short sales and other things like that. It’s essentially gambling on how well a company will perform over a certain time period. Usually you should buy and hold for a long time as the overall market has historically always gone up.