Compound interest is interest earning interest. It starts boring and becomes absurd: at 8% annual return, money doubles roughly every 9 years (Rule of 72: 72 ÷ 8). Double it 3 times and you have 8× your money — in about 27 years, without adding a cent.
This is why time in the market beats timing the market. Someone who invests $200/month from age 25 to 35 and then stops can end up with more at 65 than someone who invests $200/month from 35 to 65. The early dollars simply compound longer.
Try it yourself: open the compound interest calculator, set a modest rate, and drag the years slider. Watch the interest slice of the donut chart swallow the contributions slice. That crossing point — when your money earns more than you save — is the whole game.