The square root of the number "25" is either five or negative five. A square root of a given number is the number that when multiplied by itself yields that given number.
A square root is either positive or negative, but only positive numbers have real square roots. Two positive numbers multiplied together yield a positive number, and two negative numbers multiplied together yield a positive number as well. Knowing several square roots of numbers helps to estimate with reasonable accuracy the square roots that are not whole numbers. For instance, knowing that the square root of 16 is four and the square root of 25 is five helps establish the square root of 20 at a little less than half the difference between four and five.