A stock is a tiny share in exactly one company. If you buy a share of Nestlé, you own a small piece of the company Nestlé, if it does well, the value of your share usually rises too.
Even a large, stable company like Nestlé fluctuates in price. If it hits a rough patch, you as a shareholder carry the full risk, alone, without hundreds of other companies cushioning it. The chart alongside shows the real, current price history, not just a single number.
A simple, informal signal: if you see people literally queueing for a company, whether outside a shop, for a product, or for a service, a closer look at its stock can be worthwhile. That's no substitute for actually analysing the company's numbers, but real, visible demand is often an early sign that a business is doing well.