API stands for Application Programming Interface. It denotes a set of well defined functions or procedures that can be invoked to generate a given outcome.
In the context of WebRTC, WebRTC has a set of well defined Java Script APIs. These can be called inside any browser based web application.
In the context of cloud based systems (which use WebRTC), then an API is also the interface used to integrate with them. This is especially true with CPaaS.