What is the z-axis?
A standard x,y graph is like a 2D square. The length of the square is the x-axis and the height of the square is the y-axis, and you can use these to plot two variables against each other (eg, foot size and leg length).
However, if you want to plot three variables against each other (eg, foot size, leg length and waist size) then you can make a 3D graph that looks like a cube. As before, the length and height are the x and y-axes, but the third dimension, the depth, is called the z-axis.