The idea, in one paragraph

Compounding is what happens when the return on your money starts earning a return of its own. Put money somewhere that grows, leave the growth in place, and next year you earn on the original amount plus last year's growth. Do that for long enough and the growth becomes larger than anything you put in.

The number that makes it concrete: at around seven per cent a year, money roughly doubles every ten years. Twice over in twenty. Four times in forty. That is not a trick and it is not fast — it is arithmetic that most people meet too late to use properly.