
For example: Imagine that you are going to invest $100 per month in a certain stock. This month the stock is at $10 per share so you will be able to buy 10 shares. Next month the stock is at $11 per share so you will only be able to buy 9.09 shares. The following month the stock drops to $9 per share so you are able to get 11.11 shares. In month 4 the stock is up to $16 so you can only afford 6.25 shares. You now own 36.25 shares of stock worth an estimated $580 -- and you have invested only $400. Obviously if stocks suddenly dropped back to the $10 per share where you started, your stocks would only be worth $362.50, but the stock market has a pretty strong track record of upward movement over time. Even so, don't forget the concept of diversification... and don't invest money you can't afford to lose.
Now I always suggest consulting multiple sources of information prior to investing. Here's an example of reasons NOT to use Dollar Cost Averaging. I disagree with his reason, but it's good to listen to varied opinions.