When you try to get the area of a square [1] and know the length of a side [2] you multiply the length of one side by itself or square it.
What if you don't know the length of a side and instead only know the area of a square and want to get the length of a side? You would have to do something to the area that transforms it in the opposite direction that squaring the length of a side does. This is just like how placing 2 apples to a pile transforms a pile of apples by increasing the amount of apples and eating 2 apples transforms the pile back to the way it was. So in other words adding is to subtracting as squaring is to.... what?
Well this kind of transformation is called a Square Root.