Using your current location and direction of movement, the app tells you the direction and distance to the target-area that you selected. For example "1.3 kilometers to the left" or "800 meters ahead and to the right".
When riding in woodlands, on footpaths, or other off-road terrains it is a good idea to keep your bearings so you know which general direction you want to head in. Bike Nav lets you set a target area then runs in the background and speaks direction updates as you move. This allows you to plug in headphones (or put it in a pocket where you can hear it) and keep riding with eyes-free hands-free spoken updates, so you dont need to stop and look at your phone in order to keep your bearings. It doesnt tell you where to go and it wont navigate you around obstacles like rivers or railway lines - you are in control of where you go.
Note that this app will not work as well on devices without GPS, such as the WiFi-only iPad and iPod Touch. These devices get their location by looking at which local WiFi networks they can see, so the app will only work on these devices when you are close to urban areas and the device can see some WiFi networks. On GPS-enabled devices you can use it in the middle of nowhere.
Continued use of GPS running in the background can dramatically decrease battery life. Bike Nav uses lower GPS accuracy and less frequent location updates when you are far away from the target area, increasing the accuracy and frequency of updates as you get closer. This uses less power than if the accuracy and frequency were set to max. all the time.