Latitude and longitude readings from civilian GPS are often a little inaccurate. This app allows you to gather more precise coordinates of a location. For a couple of examples, this can be useful for adding features to OpenStreetMap or placing a geocache.
To use the app, you can start at the location you want to measure, press "Start", walk away from the location for say 30, then turn around and walk back to the location. Then continue past the location for the same distance, then return to the location of interest.
Alternatively, you can choose "Manual" mode, and add points at specific locations.