The figure and ground principle is basically organizing what is seen by differentiating the object (the figure) from its background (the ground). For example, the words on this paper represent the figure, and the white paper represents the ground.
Grouping is another way our mind uses to bring form and order to stimuli. Grouping is the perceptual tendency to organize stimuli into coherent groups following the rules of proximity, continuity, and closure. Proximity occurs when elements are placed close together. They tend to be perceived as a group. Continuity occurs when we perceive things continuous with no interruptions. Closure occurs when an object is incomplete or a space is not completely enclosed. If enough of the shape is showed, people fill in the missing information to get a complete whole object.
Gestalts psychology has proved to us that our brain does not only register information, but also, it filters that information and analyzes it to construct perceptions that make sense to us. Therefore, when a person sees a picture that they do not understand, they use ways or principles to get that picture to make sense or to get the “whole”, and those principles are figure-ground and grouping.