The name polygon comes from two Greek words. Poly roughly means "many" and gon means "corner" or "angle". A polygon is a shape with "many angles".
However, this isn't a particularly helpful definition. A better version is that a polygon is a closed 2D shape made of straight lines (e.g. not a circle or an open loop!).
Triangles and quadrilaterals are polygons and are used frequently in lots of areas of maths. Other common polygons include pentagons, hexagons and octagons, and they get their names from the number of sides they have.
There is no upper limit to the number of sides to a polygon - if we really wanted, we could draw a polygon with a hundred or a thousand sides.