Two sources of monochromatic waves are situated on either side of the origin. The sources oscillate in phase. The pattern of interference fringes is projected on a screen near the top of the display. Playing the animation will decrease the source separation and show the resulting change in the interference pattern. Actual fringes would have intensity variations that can't be displayed in the animation.
Plane waves of constant frequency move up the screen, crossing from one medium (blue) into another (gray). The wave speed in each medium is different.