What is perimiter?

Suppose you have a closed shape.  It can be anything as long as it's closed.

Now pick a point and start walking around the border of this shape.  Eventually, because it is closed, you will reach the point you started from.

The total distance you travelled along the border of the shape is called it's perimeter.

Similarly imagine snipping the border of the shape in two at some point and drawing it out into a straight line without stretching it.  The length of this line is the shape's perimeter.

Perimeter is the outside of the shape. Like the perimeter of your back yard is the fence.