The Argand Plane is significant in Class 11 Maths Chapter 5 (Complex Numbers) as it provides a geometric representation of complex numbers. Each complex number z=x+iy is plotted as a point (x,y) in a 2D plane, where the horizontal axis represents the real part and the vertical axis represents the imaginary part. This visual approach helps understand complex number operations like addition, subtraction, modulus, and conjugation more intuitively.