A stock (or share) is a small piece of ownership in a company. If a company has 100 shares in total and you own 1 share, you own 1% of the company.
Owning a share doesn't give you keys to the office — but you have a real claim on the company's future profits. If Reliance earns ₹75,000 crore next year, every rupee of that belongs, in tiny fractions, to Reliance shareholders.
Analogy: imagine your neighbourhood bakery is split into 1,000 slices. Buying 1 slice makes you a 0.1% owner. If the bakery starts making twice the profit next year, your slice becomes more valuable — and other people will pay more to buy it from you.
You buy and sell these slices on a stock exchange (NSE and BSE in India). The exchange is just the marketplace where buyers and sellers meet.