The expression "3 + 4i" represents a complex number, which consists of a real part and an imaginary part. In this case, the real part is 3, and the imaginary part is 4i, where "i" is the imaginary unit defined as the square root of -1. Complex numbers are used in various fields, including engineering and physics, to solve equations that cannot be solved using only real numbers. Complex numbers can be represented graphically on the complex plane, where the x-axis represents the real part and the y-axis represents the imaginary part. For "3 + 4i," the point would be located at (3, 4) on this plane. This representation helps visualize operations like addition and multiplication of complex numbers, making it easier to understand their properties and applications in mathematics.