Dollar Cost Averaging (DCA) is an investment approach that seeks to lower you average cost of ownership in an investment.

The way it works is you invest the same dollar amount every month or period of your choice (it could be weekly, bi-weekly, quarterly, etc.).  If you invest $50 every month you get more shares in down markets and fewer shares in up market months. 

Let's look closer at this . . .
If you invest a fixed dollar amount of $50 in June, July, and August and the stock you invested in traded $20, $25, and $30 you would own 6.17 shares worth $185 with an average per share cost of $24.32.

Now as a comparison, let's buy two shares per month in June, July, and August.  Using the same per share amounts you would own 6 shares worth $180 with an average per share cost of $25.

I don't know about you, but owning shares at a cost of $24.32 sounds better than $25.