In the animation context, multiplaning is used to create a scene with many planes placed at different distances from the camera to recreate a perspective illusion. A plane is a layer or an element. In regular flat 2D scenes, all the elements are at the same distance superposed one on top of another. So, when the camera moves, all the elements are moving at the same speed. In real life, all objects and elements are placed at different distances from our eyes so when we walk or drive past, the closest objects appear to be moving away faster. The farthest ones, like mountains, are barely moving. In animation, that perspective effect has to be reproduced manually.
In 1933, the Walt Disney Company invented the multiplane camera. They got the idea from looking at methods used in the theatre to introduce perspective into a space with limited depth. In the theatre, cut-outs and flat pieces were placed in a variety of layers in front of a backdrop which allowed actors to move through the various sets while giving the illusion of depth. Walt Disney's final design for the multiplane camera used this same concept. The multiplane camera which was set up vertically, much like a photographic enlarger, would shoot down through as many as five planes attached to vertical posts that allowed four of the planes to move independently of one another. The first two planes were used for animation, the next two were for backgrounds, and the fifth was fixed and used for sky backgrounds. The finished result produced a depth of perspective which had not previously been seen in animated film.
Multiplane camera effects can be created digitally. There is no longer a need to use the big multiplane camera. Multiplane shots are normally used to add depth to a scene to reproduce perspective effect more easily. They can also be used on a smaller scale to make elements pass in front of and behind certain objects like trees or chairs.