You "reset the start location" by touching the bottom button and the phone gps sets it to where you are standing. Next time you need to know your way back, you open up your phone again and the app will show you the direction and distance in a straight line - as the crow flies. Tap the "Distance in .." button to toggle units.
When the app starts for the very first time, the start location is Apple HQ in Cupertino. The arrow will swing around to point there, and the distance will update. You then reset the start location to where you are standing. Even when powering up the phone, this start location will be remembered - until you reset the start again.
A feature of this app is the low battery draw. Tracking by GPS is a high battery usage activity for a cell phone, and this app only updates the GPS when it is brought to the foreground. When in the background, or when the screen is off, there is negligible usage of the battery.
Forget where you parked your car? Need to get back to the start in a city but the map is too small to read? Distance by Crow shows you the way back to the starting point with a large arrow and distance in Metric or English units.
As this app is intended for outdoor use, satellite signals inside buildings are usually weaker and can be inaccurate.