An API is, general terms, it is a set of clearly defined methods of communication between various software components.
Cachly obtains data from geocaching.com using the API they supply. This means that this app is limited to using only the data that is available through the API. There is data on geocaching.com that is not available through the API. Cachly is precluded from obtaining this data through other means in terms of am agreement with geocaching.com.