system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What IS the "stock market" and how does it work?
gpt
It's a place where ownership of companies is traded.  It works like any other market (it's structured as open outcry which is chaotic on the surface but is a very good way to transfer information widely and rapidly).  

So if you start a company called Charlie's Surfboards and need money for a factory, you can go to investors and sell a portion of the ownership in the total company in exchange for the money to build the factory.  The idea being that with a new factory owning half the company will make you more money than you would continuing out of your garage, and investors get a return on their money because they'll also own half the company.  That works well, until one of the investors want's to invest in Bulksalty Snacks Company and hasn't earned enough from Charlie's Surfboards to buy any.  

So the investor has a desire to sell, but to do so is sort of like selling a home he needs to find someone who likes the surfboard business and has enough money to buy the 50% stake.  

With a stock market, once Charlie's Surfboard goes public his 50% stake is a large number of smaller shares that because they're small allow lots more investors to afford.  So the investor can simply sell some of his Charlie's Surfboard shares and buy Bulksalty Snack shares.  

When he does so, the prices will update based on the new trade and people may act on the information (a large investor is selling something and buying something else, why, what does he know, etc).