An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software.
Cited from https://en.wikipedia.org/wiki/API
Application: a computer software package that performs a specific function directly for an end user or, in some cases, for another application.
Programming: is the process of performing a particular computation
Cited from https://en.wikipedia.org/wiki/Computer_programming
Interface: a point where two systems, subjects, organizations, etc. meet and interact.
In general, APIs help systems communicate and transfer data from one endpoint to another.
Microsoft Graph is Microsoft's API infrastructure and more information can be found at https://learn.microsoft.com/en-us/graph/
----------------------------------
If this is helpful please accept answer.