The concept of buying dips is based on market fluctuation. Because the market is volatile, any given dip in prices should eventually rise back up. By purchasing stocks right after a dip, investors are essentially buying shares at a discounted sale price.
Like all trading strategies, buying the dips is not a sure thing, because some stock price drops are due to negative changes in the underlying company's fundamentals. For example, investors who followed this strategy around the bursting of the dotcom bubble may have lost a lot of money because many internet companies lacked a proper revenue-generating business model.