A Fabric API is a set of programming instructions that allow software programs to interact with each other. This can include anything from simple tasks, like reading or writing data, to more complicated actions, such as sending a message or making a request.
Fabric APIs are extremely common and are used by a wide variety of applications. For example, a social media site might use a Fabric API to send messages to a user's friends, or a video game might use one to download new levels.
API stands for "Application Programming Interface."