The black line shows you the result of the red and blue waves being added together. This can look confusing but try and get a feel for it. when you show the grid the solid lines represent nodes and the dashed lines the position of the anti-nodes. Key points to remember are listed below.
Stationary or standing waves occur when identical waves moving in opposite directions meet. Certain points have no displacement, these are called nodes. Other points, the anti-nodes have maximum displacement. Node to node distance = half the wavelength Node to anti-node distance = one quarter the wavelength