I think it's trying to get you to find the side length of the square by using a square root on the area.
Assuming that's the case;
Remember that a square has equal side lengths all around, otherwise it would just be a rectangle. You can actually take the square root of a square to get the side length (hence the name square root). A square root is what number you must multiply by itself to get the root, so for example the square root of 25 is 5 because you can multiply 5 by 5 to get 25. The square root of 100 is 10, because you can multiply 10 by itself to get 100. So you would write this as
, which simplifies down to 10.