Let's explain this by using an example. Let's take the square root of 4.
sqrt(4)
You'd think the answer is 2, and you are correct, but the answer is also -2.
Remember that a square root means that the two numbers must be the same, and multiply back up to 4. In this case, the number is 2.
2•2=4
BUT
-2•-2 ALSO =4
therefore there are two square roots...
2 is a square root
AND
-2 is a square root.
This goes with all positive numbers!
I hope this helps! Best wishes :)