When indexing, most people like to invest the same dollar amount of money into an index every month. That way, you never buy all of your stock at the very top of the market. By buying a stock or index/ETF at different times, you are allowing the wiggles of the stock to smooth themselves out, since you are always buying at a different price. By doing this, you end up getting a pretty good "average price.” That is why this practice is called "cost averaging.”
