Modified from ETH Top, Mid and Bottom Cap Bands [jcray], which in turn was modified from BTC Top and Average Caps by aamonkey.
The top cap (red line) can be seen as an absolute market top that may never be significantly exceeded and so is a good indicator to sell a position, or enter a long-term short-sell.
The mid cap (yellow line) can be used as an indicator for market sentiment. Above the mid cap, the market is in a bullish environment, and below it, the market is in a bearish environment.
The bottom cap (blue line) can be seen as an absolute market bottom that may never be fallen short of, and so is a good indicator to enter a long-term buying opportunity, or exit a long-term short-sell.