Theory:

Polygon is a closed two-dimensional shape consisting of only line segments. It does not contain a curved shape. We can also say, polygons are simple closed curves.
A three sided polygon is called a triangle. It is formed by joining three line segments.

A four sided polygon is called a quadrilateral. It is formed by joining four line segments.

A five sided polygon is called a pentagon. It is formed by joining five line segments.

A six sided polygon is called a hexagon. It is formed by joining six line segments.

A seven sided polygon is called a heptagon. It is formed by joining seven line segments.

A eight sided polygon is called an octagon. It is formed by joining eight line segments.

It goes on. We can also form $$n$$ sided polygon.

Let us see some figures, which are not polygons.
Example:
1

The shape is made up of line segments, but it is not closed. So, this is not a polygon.

2.

This is a closed shape but not simple. So, this is not a polygon.

3.

As per the definition, a polygon does not contain a curved shape. So, this is not a polygon.

4.

This is a three dimensional figure. So, this is not a polygon.