Broadly speaking, market breadth measures if any rally in the market or fall in the market is broad based or restricted to just a handful of stocks. Market breadth has a plethora of interpretations. It can refer to the number of stocks, or sectors or themes. But the moral of the story is simple. Never bet on a market direction that is not supported by breadth. Market breadth gives the legs for the market to stand on. In fact, the final and decisive confirmation of a market trend is always the breadth. Broader the trend, more affirmative the direction! Typically, higher market breadth is indicative of more active participation from mid cap and small cap stocks which are indicative of broad based participation in the market.