The square root of -1 equals i. This "i" is called an imaginary number. If you combine this imaginary number with a real number, for example, 2+i, it's called a complex number. So the other numbers can't see the square root of -1 because it's an imaginary number. And it's a "complex friendship" because 8 and i (a real and an imaginary number) can form a complex number.