Contrary to what some people might tell you, imaginary numbers are not numbers that only exist in the brains of weird people. But this is not what people mean when they talk about imaginary numbers.
Imaginary numbers are numbers that can be written as a real number times i.
Well, the real numbers are all the positive numbers, negative numbers, and zero. And it's NOT a real number. So we can make an imaginary number by taking a real number like 5 and multiplying it by i. That gives us 5i. Browse the Dr. Math archives to find answers written for a variety of levels.