Application
Programming
Interface.
Translation to English: a set of Programming Libraries that allows a Software Developer's computer programs to take advantage of different facilities provided by other software. In the particular case GJ mentions, Windows has a huge and complex API that allows applications to use a lot of things Windows provides: access to the GUI, access to the different devices Windows handles etc.
Say you want to write a program to talk to your smartphone, grab the SMSs, and display them in your computer screen. Instead of writing the code to talk to the phone on the USB port, there are APIs provided both by Microsoft and the Smart-Phone software manufacturer that will do that, Microsoft also provides APIs for the program to talk to the mouse, screen, keyboard etc, so your program ONLY has to concern itself on asking all those devices through the APIs to do their job while your program does what it was intended to.
The idea is that you there's a standard way of doing things that will both reduce the complexity of your program and also make sure (theoretically at least) that it works with future versions of Windows and the Smart-Phone software.
SO, what do you do in that institute?, see Dark Matter is composed of Pancakes or Pancake-to-neutrino conversion are the way the Great Attractor caused the New York Blackout of 1977?