Squares are regular polygons. Certain triangles are regular. The sides are all the same and the angles are all the same.
A regular polygon is also a pentagon, a hexagon, a heptagon, an octagon, a nonagon. It is a closed shape made up of straight lines connected by corners or vertices (vertex). All the lines are the same length and all the corners are the same number of degrees. The lines don’t cross one another.
Here is another look at regular polygons.