In short: Regular saving means putting aside the same amount every month or week, which over time — with interest on the interest — grows to more than the simple sum of the payments.
Simply put: Small regular amounts plus time add up to a lot. Doubling the number of years more than doubles the final sum, because interest also earns interest. That is why starting early is cheaper than starting late. The return, however, depends on where the money is and how much risk it carries — compounding only works if the return is positive after fees and inflation.
Example: €100 a month at 5% a year (interest added monthly): over 10 years you pay in €12,000 and have €15,528.23; over 20 years you pay in €24,000 and have €41,103.37.
Watch out: The example is maths, not a promise: a deposit may pay less, and an investment has no guaranteed return. Compare with inflation.
Source: Stango & Zinman, "Exponential Growth Bias and Household Finance", Journal of Finance, 2009 — https://doi.org/10.1111/j.1540-6261.2009.01518.x · GFLEC — https://gflec.org/education/questions-that-indicate-financial-literacy/