The McClellan Summation Index is a long-term version of the McClellan Oscillator. It is a market breadth indicator, and interpretation is similar to that of the McClellan Oscillator, except that it is more suited to major trends.
Usually, a small number of stocks making large gains characterizes a weakening bull market. This gives the perception that the overall market is healthy, but in reality it isn't, as rising prices are being driven by a small number of stocks. Conversely, when a bear market is still declining, but a smaller amount of stocks are declining, an end to the bear market may be near.