Learn to tell the difference between real and imaginary numbers.
Real numbers include all rational numbers (numbers that can be written like fractions) and all irrational numbers (numbers that cannot be written like fractions). Each of the infinite number of points on the real number line represents a real number.
Believe it or not, there are other types numbers that aren’t on the real number line. They're called "complex numbers," and they're made by adding together a real number (like the ones we know and love) and something else called an "imaginary number."
What do imaginary numbers look like? Well, they’re sort of like real numbers, but they’re definitely stranger. We won’t go into all the details here, but imaginary numbers are all multiples of something called the imaginary unit, which we write with the letter i. And--here comes the strange part--this imaginary unit has the unusual property that its square is a negative number. In particular, i^2 = –1...which is definitely something you don’t see with real numbers!
For more, see What are Real Numbers?