A square root of a number is a number that, when it is multiplied by itself (squared), gives the first number again. For example, 2 is the square root of 4, because 2×2=4. Only numbers bigger than or equal to zero have real square roots. A number bigger than zero has two square roots: one is positive (bigger than zero) and the other is negative (smaller than zero). For example, 4 has two square roots: 2 and −2. The only square root of zero is zero.
The sign for a square root is made by putting a bent line over a number, like this: . We say "the square root of 4" (or whatever number we are taking the square root of).
It is not really known where the square root symbol comes from, but some people believe that it was from the letter r, which is the first letter of the Latin and German word radix. Radix means square root.