Zero has one square root which is 0. Negative numbers don't have real square roots since a square is either positive or 0. The square roots of numbers that are not a perfect square are members of the irrational numbers. This means that they can't be written as the quotient of two integers.
The meaning of this idiom is that you should never feel hopeless when times get hard or difficult because difficult times always lead to better days. The difficult times are like dark clouds that blocks the sun. When we look at the edges of the cloud more closely, we can see the sun shining there like a silver lining.
