Mathematicians from time to time like to use terms that in the context of mathematics has very specific meanings but to the rest of the world has a completely different meaning.
"Real Numbers" is one of these terms. In first year uni math (or 4 unit math) you'll come across what are known as imaginary number which, when combined with real numbers form complex numbers. If this is confusing, then I wouldn't worry as you don't really need to know the difference just yet.
Using InteGrand's example: the quadratics x^2 + 1 = 0 doesn't have any "real" solutions, it does have an "imaginary" solution of x = i (where i is the square root of -1 but again, you don't really need to know that just yet).
So next time you see the term "real" in a maths question, understand that mathematicians are extremely thorough when asking questions. The terms real, defined and undefined all have specific meanings but for someone in high school you can use the everyday meanings and they will work just fine.