API stands for Application Programming Interface, which is a set of protocols, procedures, and tools that allow interaction between two applications. An API call, or API request, is the process of a client application submitting a request to an API and that API retrieving the requested data from the external server or program and delivering it back to the client. In other words, an API call is a message sent to a server asking an API to provide a service or information.

API calls are essential tools in the development of software applications as they allow different applications to talk to each other and exchange information or functionality. APIs enable developers to use other technologies and data as building blocks in their applications, which simplifies software development and enhances productivity.

To make an API call, the client application needs to know the Uniform Resource Identifier (URI) of the server or external program whose data it wants to access. The client application then adds an HTTP verb, includes a header, and includes an API key or access token. Once the request is assessed and forwarded to an external program, the program passes back data to give the user a response.

API calls can be used for various purposes, such as retrieving data or triggering specific functions. They can also be used maliciously, so its crucial to ensure that API calls are properly secured to prevent potential vulnerabilities.