An API is the user-facing interface of a system. It's the abstraction that allows you to interact with that system. It might sound a lot more complex than it is, so think of the buttons of a coffee maker; they represent the API of the coffee maker. While the expression has its roots in software, it isn't limited to it. It's a mental model for designing interfaces. Start by asking how the users are going to use the thing you want to make. I remember reading a story about a medical device in the design, where they ended up remaking the product from scratch when they realized medical staff was going to be using them with one hand, the other being busy with some other piece of equipment. Context matters.