Let the expression on the right be $g(x)$. The method converges (sometimes) to the solution of the equation $x=g(x)$.
The cobweb diagram illustrates the movement from first guess $x_0$ to second guess $x_1$ etc.
Draw a line from $(x_0,0)$ up to $(x_0,x1)$.
Then across to $(x_1,x_1)$.
Then up or down to $(x_1,x_2)$.
Then across to $(x_2,x_2)$.
Etc