It doesn't exist, basically, since 10 squared is 100, and -10 squared is also 100.
Some random mathematicians invented an imaginary number, i, which is the result of the square root of -1.
[tex] \sqrt{-100} [/tex] -> [tex] \sqrt{-1} * \sqrt{100} [/tex] -> [tex]i * 10[/tex] 10i
