Sale of the product is done many times before the first purchaser acquires it!

I’m a complete newbie, like I just searched how to make money trading stocks kinda newbie.

I read on a blog, "in the stock market, the product which is the subject of the deal is sold many times whilst still in the possession of the first seller and before the first purchaser acquires it."

I used to believe that you open an online account and then buy stocks of the companies or commodities that you believe that their prices will go up.

How can I buy and sell something that I haven’t possessed yet? What does that quoted sentence referring to? I wanna understand.