API
An interface enabling one piece of software to interact with another — adjacent professional vocabulary a designer encounters, not a design concept in its own right.
In practice
An API is a set of agreed requests one program can make to another, such as asking a weather service for today's forecast and receiving structured data back. Designers rarely build APIs, but they meet them often: a dashboard design depends on which data the API actually provides, how fast it arrives and what happens when it fails. Asking developers what an API returns before drawing screens avoids designing fields that cannot be filled.
Often confused with Interface
A user interface is something people operate; an API is an interface for software to talk to other software, with no visual surface at all.
Try this
Read a public weather service's documentation and list which data fields a forecast screen could realistically display.