Compound interest is the addition of interest to the principal sum of a loan or deposit, or in other words, interest on interest. It is the result of reinvesting interest, rather than paying it out, so that interest in the next period is then earned on the principal sum plus previously accumulated interest. Compound interest is standard in finance and economics. - Wikipedia
The default example above is an example of you saving $250/month when you start earning money into your 401k or IRA and investing it to a mutual fund like VTSAX or VFINX for the next 30 years.