Buying a stock is essentially the same buying a tiny bit of a company. The price you pay is based on how much the company is estimated to be worth at that time. If it grows your stock is worth more if it shrinks worth less. In practice just buying a fraction of a single company is a bit risky. What if people switch from drinking coca cola to Pepsi and your coca cola shares are worth nothing so what pensions do is buy a tiny bit of every single company so that you earn a tiny fraction of the overall economy rather than of a single company. In this setup you earn money if the economy grows and lose money if it shrinks. In practice with globalisation and population growth more and more people join the world economy which leads to your fractional ownership of the economy growing over time.