Market Cap = current share price × total number of shares. It's the total market value of the company.
Reliance at ₹1,300 × ~677 crore shares = ~₹17.6 lakh crore. That's the price you'd pay to buy every share of Reliance today.
Rough Indian labels: Large-cap (top 100 by market cap, roughly ₹20,000 cr+), Mid-cap (101st–250th), Small-cap (below that). Large-caps tend to be more stable; small-caps can grow faster but swing harder.
Analogy: if a stock is one slice of a pizza, market cap is the total price of the whole pizza — regardless of how many slices you cut it into.