Dollar-cost averaging (DCA) means investing a fixed amount on a regular schedule — weekly, monthly, whatever — regardless of whether prices are up or down that day. It trades the (impossible) goal of timing the market for a simple, repeatable habit.
Why DCA works psychologically as much as mathematically
Markets go up and down unpredictably in the short term. DCA removes the emotional decision of 'is now a good time to invest' — you invest on schedule either way, buying more shares when prices are low and fewer when prices are high, which averages out your cost basis over time.
A worked example
If you invest $300 a month for 15 years at an average 8% annual return, you'll have contributed $54,000 of your own money — but your projected balance will be meaningfully higher, because each contribution has had time to grow before the next one arrives.