An API is short for Application Programming Interface. This is a software interface that helps computers access data and tools built and provided by an organisation. There are thousands of different APIs, all of which sever a specific function, or support specific commands, syntax and protocols.
While there is some movement to open up APIs in certain sectors (making them public), most of them are gated and require permissions to access. As such, APIs are considered secure and trustworthy. An example of the use of an API is the plugging-in of a Google feature into a website, in order for that website to perfectly support software built elsewhere.