Compound interest is what separates saving from investing. In a non-yielding account, €1,000 stays €1,000 in ten years (minus inflation). With compound interest, generated interest is reinvested and produces interest on interest.
Basic formula: FV = PV × (1 + i)^n, where FV is future value, PV is present value, i is the rate per period and n the number of periods. Long term, a low rate over many years beats a high rate over few years.
It is the engine behind index funds, pension plans, compound deposits and most of the advice to “start saving early”.