The principal square root of ##64## is:
##sqrt(64) = 8##
The other (non-principal) square root is:
##-sqrt(64) = -8##
##64## has two square roots namely ##8## and ##-8## since:
##8^2 = (-8)^2 = 64##
When we say the square root what is usually intended is the principal square root which in the case of the Real square root of a positive number is the positive one.
Any non-zero number ##n## has two square roots. In order to distinguish between them we call one the principal square root which in the case of ##n > 0## means the positive one.
##color(white)()##Complex footnote
If ##n < 0## then it has two Complex non-Real square roots: ##+-i sqrt(-n)## In this case we call ##i sqrt(-n)## the principal square root and ##-i sqrt(-n)## the non-principal one. For example: ##sqrt(-64) = 8i## is the principal square root. ##-sqrt(-64) = -8i## is the other square root. Note that ##8i## is not positive. Unlike Real numbers Complex numbers are not ordered but for pure imaginary square roots we choose the one with the positive imaginary part and call it principal.