Market breadth means how many, out of the total number of stocks listed on a particular market are following the general trend of the market. In a broad bull market, the large majority of stocks in the market are making price gains. In a broad bear market, the large majority of stocks are declining in price.
Market breadth is often a leading indicator of a change in trend. If the breadth in a market starts to thin out, i.e. fewer and fewer stocks are participating in the general trend, it becomes more likely that the whole market will change direction, in other words, make a trend reversal.
Very often only a few blue-chip or large-cap stocks account for most of the movement in the main headline indexes like the Dow Jones Industrial Average or the Standard and Poor’s 500 Index. These indexes can continue in one direction while market breadth is declining. This is a measure of divergence, signaling a likely trend reversal.