Strip away the screens and jargon and a market is just this: people who want to buy meeting people who want to sell, and agreeing on a number. Every price you've ever seen quoted — a stock, a currency, an ounce of gold — is simply the most recent agreement. Nothing more mystical than that.
Price is an agreement, not a fact
A share of a company doesn't "have" a price the way a table has a length. It has the last price two strangers agreed on. At any moment, would-be buyers are saying "I'll pay this much" and would-be sellers are saying "I'll accept that much." When someone gives in and crosses the gap, a trade happens — and the number they agreed on becomes, for one instant, the price. Then the argument resumes.
So why do prices move?
Because the crowd's opinion moves. Good news about a company means more people want in and fewer want out — buyers must offer more to tempt the reluctant sellers, and the agreements happen at higher and higher numbers. Bad news runs the film in reverse. No committee decides this; it emerges from thousands of separate decisions, which is why prices move constantly, sometimes violently, and often before the news is obvious. When you hear "the market thinks the company is in trouble," that's shorthand for: the current agreements are happening at lower prices than yesterday's.
Here's the part worth remembering forever: every trade has two sides who disagree. Whoever sold you your shares preferred the money; you preferred the shares. One of you will look smarter later. All of trading — every course on this site — is about trying to be on the better-informed side of that disagreement more often than chance.
Why markets exist at all
It's fair to ask what all this arguing is for. Two things, and they're genuinely useful: markets let businesses raise money to grow (people buy ownership pieces — Lesson 2), and they let anyone convert things into cash and back at a public, honest number. That second property — called liquidity — is quietly precious: a house takes months to sell at a guessed price; a share sells in one second at a known one. The trading you'll learn about lives entirely inside that machinery.
Check yourself: a stock's price just fell 5% in a minute. What actually happened?
Not "the company lost value" by decree — what happened is that the agreements changed: sellers wanting out badly enough accepted lower and lower offers from buyers who would only bid cautiously. The price is the trail those agreements left. Why the sellers were so eager is the interesting question — and it's what the rest of trading education is about.