You should open this settings screen directly after install. You can set your currency, threshold values, display style and colors. Simply tap on the options (lower left of each one) to open a settings dialog. If there are more options then fit on the screen, scroll by swiping up or down.
Each change you make is immediately reflected on the screen of your device.
You set the low rate in order to get an alert when the Bitcoin rate drops to or below a certain value. The high rate works opposite.
This will help you to pick the right moment for buying or selling.