The number of square units in the polygon gives the area of the polygon. It is easy to distinguish between perimeter and area. Just think a perimeter as a fence length you need to cover the yard, the space inside the yard is the area. Area has both length and width and hence it can be measured in square units and expressed as square inches, square feet or square meter.
Formula for the Area of a Rectangle = $ L \times W $
Where L is the Length and W is the Width.