Area is measured in square units.  They are called square units because they are little squares that are one unit long and one unit wide. Those units could be inches, centimeters, feet, yards, or any other linear units of measure.

To find area of this rectangle, we count the number of square units in one row.  We then count the number of rows needed to completely cover the rectangle, with no gaps or overlaps.

There are 6 square units in one row and 4 rows are needed to cover the rectangle. Therefore 24 square units are needed to cover the rectangle.

The formula for the area of a rectangle is A = l ⋅ w. In this case, l = 6 and w = 4  using the area formula, the area is 6 x 4 = 24 square units or 24 units².

When the area is not a rectangle or a square, use square units and parts of square units are used to measure the area.  Look at the area of the circle below.

Many of the square units are only partial units.  If you put the pieces together, they will form whole units. 

The formula for area of a circle is A = πr² where A represents the area of the circle and r represents the radius, or distance from the center to the outside of the circle and π represents the ratio of the circumference of a circle to its diameter. 

In every circle, the circumference is a little more than three times the diameter or 3.14.