A solution to the simple second-degree equation
x2 + 1 =0
can not be found along the line of real-numbers.
Therefore it was necessary to invent a fictive number i such that i2=-1.
i.e. the imaginary numbers making it possible to take the square root of negative numbers.
They are represented along an axis horizontal to the axis of real numbers.
A combination of a real number and a imaginary number is called a complex number.
z = 2 + 3i.