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If you invest 100 € a month with an average annual return of 7 %, in ten years you'll have contributed 12,000 € and you'll have around 17,300. In thirty years you'll have contributed 36,000… and you'll have more than 120,000. That gap is compound interest: your interest starts to earn interest of its own, and the ball grows a little faster with every turn.
The detail almost no one takes to heart is that the decisive variable isn't the amount, it's time. Someone who starts at twenty-five with little usually ends up ahead of someone who starts at forty with a lot, because the final years of the curve do nearly all the work.
The practical takeaway is far from epic: start earlier, contribute always, don't interrupt. Compound interest doesn't reward the brilliant; it rewards those who never get off.