The expression "z = a + bi" represents a complex number, where "z" is the complex number itself. In this equation, "a" is the real part, and "bi" is the imaginary part, with "i" denoting the imaginary unit, which is defined as the square root of -1. Complex numbers are used in various fields, including engineering and physics, to solve problems that cannot be addressed using only real numbers. Complex numbers can be visualized on a two-dimensional plane known as the complex plane. The horizontal axis represents the real part "a," while the vertical axis represents the imaginary part "bi." This graphical representation helps in understanding operations like addition, subtraction, and multiplication of complex numbers, making them a valuable tool in mathematics and applied sciences.