The number 64 has two square roots: -8 and 8. This is because 8 squared, or 8 times 8, is 64, and -8 squared, or -8 times -8, is also 64. Its principal square root is 8.
Every positive number has two square roots, one positive and one negative. However, each positive number has only one principal square root. When people use the phrase "the square root," they often mean "the principal square root." The principal square root is non-negative. Sixty-four is also a square number. This means that it is an integer that is a square of an integer, in this case, 8.