A polygon (also called a "face") is a flat/planar surface made up of at least 3 vertices that form a "triangle" (or a "quadrilateral" if 4 vertices) which, when combined with many other polygons, create a primitive.
There is currently no way to create individual polygons in SL. However, there are ways to mimic their properties using certain primitives. Jeffrey Gomez uses a cube to simulate a triangle in his 3D Model Importer.