system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does the stock market work and what exactly is the DOW.
gpt
Shares of a company (sometimes called "stock in a company" or just "stock") are ownership of specified percentages of a company.  The "stock market" is like a yard sale - people that want to buy shares ask people that own them what the price is, and they haggle.  If somebody at the next table over just sold something for $10, you'd want to pay about $10 for it too, right?

Now, if you expect to be able to sell it to your buddy later for $11, you might offer $10.05 so the guy will sell to you rather than all the other guys offering $10.  Others know that, and might be offering $10.10.  It depends on what they're comfortable paying and how much they think the price will rise before they sell it.

Or maybe you don't think it's worth $10.  There might be not a lot of people buying but several people selling.  You'd be crazy to pay more than you had to, right?  So you shop around and find somebody willing to sell to you for $9.75.

That's the stock market.  The DOW (or "Dow Jones Industrial Average") is an "index" or fund made up of a group of 30 large US companies.  There are other indices, like the S&P500 (500 largest companies traded on the NY Stock Exchange or NASDAQ Stock Exchange).