It compares ETH/GBP and BTC/GBP with each other to determine if the ETH/BTC price matches.
From it you can predetermine whether the value of the asset you are watching is overpriced or underpriced and therefore predict the candle to follow.
When watching ETH:
If the value is high, the value of BTC is too high and ETH should raise to match in the next bar and vice versa.
When watching BTC:
If the value is high, the value of BTC is too high and should drop on the next bar and vice versa.
The Upper and Lowers are set at double the percentage fees on the exchange you use. This just helps in recognising beneficial trades.