A square root is basically a number that when multiplied by itself gets another number.
Any number can be a square root because you can multiply any number by itself to get another number. (or the same number, if you were to multiply 0 by 0 or 1 by 1)
For example, 2 is a square root of 4, because when you multiply 2 by itself, you get 4.
3 is the square root of 9, because when you multiply 3 by itself, you get 9.
4 is the square root of 16, because when you multiple 4 by itself, you get 16.
Now what is the square root of 36?
6, because when you multiply 6 by itself, you get 6.
So, 6 is the square root of 36.
-2 is also the square root of 4 because when you multiply -2 by itself you also get 4.
-3 is also the square root of 9 because when you multiply -3 by itself you get 9.
So each positive number has a positive square root and a negative square root.
4 has 2 and -2 as square roots.
9 has 3 and -3 as square roots.
Also, SQUARE ROOTS NEVER MAKE NEGATIVE NUMBERS. You can never multiply a number by itself to get a negative number. (imaginary numbers, yes, but that is something else)
The way I explained it is kind of confusing but you can use Khan Academy (a free site) to learn more about them. They are actually quite easy to understand.
Click here to watch the Khan Academy video on square roots.
You can use square roots to find sides of a square, for example. The area of a square is side times side.
If you only had the area of a square and wanted to find a side, you could use square root.
for example, if the area of a square is 4, you know that the side length is 2 because 2 times 2 is 4.