A polygon is a closed shape with straight sides. Rectangles, triangles, hexagons, and octagons are all examples of polygons.
The word polygon comes from the Greeks, like most terms in geometry, which they invented. It simply means many (poly) angles (gon). A polygon can’t have any curves or any gaps or openings in its shape. If you want to describe something as angular and closed, like, say, the Pentagon, or a stop sign, you could call it "polygonal."