You have the followig equation:
y² = 100
In order to solve the previous equation, you take into account that in an equation, when you operate in some way (addition, subtraction, division, exponentiation, radication, etc) to one side, the same operation has to be made in the other side.
In this case you have to eliminate the exponent 2 of the y variable to solve the equation. Here, you can apply square root both sides, because in such a way you eliminate the exponent 2. You proceed as follow:
y² = 100 apply square rooth both sides
√(y²) = √(100) consider that 100=10²
√(y²) = √(10²)
y = 10
Hence, the solution to the given equation is y = 10