Common fate is a gestalt concept the refers to the visual tendency to group shapes together if the shapes move in a similar direction. The shapes do not have to exhibit similarity or exhibit proximity.
In the animation below, look at the groupings when there is no motion. The circles create one group due to similarity and the squares to the right create a second group due to similarity. All of the figures can be perceived as one large group due to proximity. But when the shapes begin to move, the groupings are quite different.
Look at the image below and determine what would be a group, then watch the animation below the image.
In the static image, it is impossible to distinguish groups. However, when motion is visible and common fate is exhibited, a group of orange circles becomes apparent. Common fate allows the perception of figures that would be imperceptible without motion.