A square root of a number is a number that, when you square it, equals the number you started with.
So for example, the square root of 4 is 2, because the square of 2 is 4.
The square root of 10 is approximately equal to 3.162.
However, it's also true that the square of -2 is 4. So why isn't -2 the square root of 4? That's because the square root function is specifically defined in such a way that it only refers to the positive value.
Any non-negative real number can have a square root that is also a real number. But the square root of a negative real number requires the creation of a different kind of number, referred to as an "imaginary" number. Imaginary numbers are not things that you come upon in everyday life. However, imaginary numbers do follow consistent rules, and there are many things in nature that can be described by math involving imaginary numbers.