Market capitalisation ("market cap") is a company's share price multiplied by the number of shares outstanding. A company trading at $50 a share with 2 billion shares outstanding has a $100 billion market cap — that is the market's collective valuation of the entire company, not just one share.
Market cap is what separates a $500 stock from a $5 stock in terms of actual company size — price alone tells you nothing about scale, since a company can split its shares to lower the price without changing what the business is worth. Companies are commonly grouped by market cap into large-cap (roughly $10 billion and up), mid-cap, and small-cap, and that grouping correlates loosely with volatility and how closely a stock tracks the broader market.
It moves every time the share price does, in real time during market hours, which is why the market cap figure on a symbol page is a live number, not a static fact about the company.