Open Platform (API)
An Open API (often referred to as a Public API) is a publicly available application programming interface that provides developers with programmatic access to a proprietary software application. APIs are sets of requirements that govern how one application can communicate and interact with another. They also allow developers to access certain internal functions of a program. In the simplest terms, an API allows one piece of software to interact with another piece of software. They may be used by both developers inside the organization that published the API or by any developers outside that organisation who wish to register for access to the interface.
Open API vs Open Source
An open API is one that is openly an freely available for anyone to use. Open APIs make a lot of sense on the web, where an open web API can be used to let third party software developers integrate pieces of your web site into theirs. While, the core idea behind open source is to give away the source code to your software so that anyone can study, fix and improve it. This tends to produce better software and happier developers and customers. However, the idea of open source goes against decades of software distribution business models that are based on keeping source code private and selling you software that you can’t modify.
Below is another reference and well explained about what an Open API means
Open Source - decentralized software development
Open Api - decentralized business development