Polygons are simple shapes that create the geometry on a 3D model with a 3D software. These polygons are used to make any object you can think of, characters, environments, objects you the 3D model then polygons exist in it. When you see this model within a game you cannot see these polygons, the model looks smooth and rendered out instead of a selection of shapes.
API,
API or Application Programming Interface is a bunch of tools for building software applications> A good API is when the software allows the user to create software with ease by supplying the necessary building blocks. Then the user will put these blocks together. API's are open to the public to use for free.
Graphics Pipeline,
Graphics Pipeline is the process of making a 3D image or model look like a 2D raster image. It does this by flattening the image to a 2D form.
