The purpose of the applet is to highlight phase difference. Two SHM's along the x-axis of differing amplitudes and frequencies are being added. The motion of the cyan particle x1 = A1 cos ω1t and the yellow particle x2 = A2 cos (ω2t+δ) the individual oscillations. The red particle (when show sum check box is ticked) shows the resultant motion. The two force vectors causing the motion are also shown.
How the phase difference between the two affects the resultant oscillation is shown visually. The reference circle drawn on the right, highlights the phase difference. The radius of the reference circle is not to scale.