In order to simplify a square root, you need to find a number that multiplied times itself equals the number in the radical. Now 7×7 is 49. Also, -7×-7 is 49 as well. But there is no number times itself that makes -49. No number times itself makes a negative number.
(You will, in time learn imaginary numbers in order to simplify this, but that can be for another day!)
