Its a math joke. the letter "i" represents the square root of negative 1, which doesn't exist, so its denoted as "i", for imaginary number. the number "i" is therefore not a real number, as it doesn't exist on the real number plane, it exists on the axis which is orthogonal to the real number plane. The other symbol is called pi, its called an irrational number, which means it cant be expressed accurately as a fraction or as a number, it can only be estimated since it has infinite decimal points. The picture then combines that with the cliches of "get real" and "be rational" and forms a math joke.
