[webmobdev]

API just defines how another software can connect and interact with your software application. An API being "open" just means that the documentation of the way to connect and interact through an API is publicly available. The "Open Source" terminology however applies to source code of a software. If someone claims an API is "open source", it means the source code of the application providing the API is publicly available. In this case, the API is offered as a service and the software application that actually exposes and executes the API is on a server. That is why people are asking if the API is open source, where is the source code of the backend.

Edit: I can see now that someone has posted a link to the repository of their backend / API source code too. Many must have missed that earlier, and hence the query.