This theorem is only used in right angled triangles. It can be used when you know two of the sides and you want to work out the third.
Right angle - 90o angle
Hypotenuse - the longest side in a right angled triangle, it is always opposite the right angle.
Square of a number - the number multiplied by itself
Square root of a number - the number that was multiplied by itself to get the given number as a result.