KB User's Guide - API - What is an API?
An Application Program Interface (API) is a set of routines, protocols, and tools for building software applications which specifies how software components should interact. They essentially provide a means to receive and/or send data to and from an application. Because the data is stripped of information related to its presentation, it can more easily be consumed by other applications.
For more background the KB's APIs, you can watch the recording from the 2016 KB User Group Meeting, led by Weizhong Wang, the KB's Technical Lead. The specific section relating to APIs starts at 12:24 in the recording.
FreeCodeCamp also has a helpful in-depth explanation of how APIs work if you are interested in learning more.