An API, in simple terms, is a method by which two software components can communicate with each other, and provides the software developer with a set of routines and tools they can use to build new software on.
It should be noted that the API is a tool with which a qualified programmer may build new, custom software integrated with the PCR database, but cannot be used for altering the PCR software itself, which remains solely under the control of Main Sequence.
The documentation for the REST API can be found at https://www.pcrecruiter.net/APIDOCS_V2/