when you take the square root of a number, for example √16 , you know that the answer is 4. Why is so? It is because 4·4 = 16, when taking √16 we look for a number that when multiplied by itself we get 16.
Another way to write 4·4 is 42. Thus, √42 = 4.
The same for all other positive numbers. √32 = √9 = 3 or √52 = √25 = 5... and so on and so forth.
Since this is true for any positive number, we can use the variables (either x or y in this case) to represent this fact.
√x2 = x.
You can also say, that for every positive number x, √ and the power of 2 cancel each other.