The following graphic shows the relationship between length, width, and area of a square. If you think of this as a series of rows that make up the area it may help to you to understand. Let's assume that we have a square with a width of 100 feet and a length of 100 feet.
Lets take each row which are 1 foot each and look at what we find. 1 row that is 100 feet long has an area of 100 square feet. (length of 100 times width of 1 = 100)
Add a second row and we have two rows. What happens to the area. 2 1 foot rows that are 100 feet long gives an area of 200 square feet.
The entire square then has an area of what? 100 x 100 = 10000 square feet.
Hope this helps.