The Plan View tab allows you to see your structure or design on a layer-by-layer basis. The current layer is displayed using black squares (or textured squares for more complex designs) while the layer below can be seen shaded grey. This allows you to accurately and easily position the blocks needed for the current layer of the design.
There are two sliders present on this view. The 'Layer' slider to the left of the plan allows you to move up and down through the various layers of your structure. As you move the slider the plan view dynamically updates to show the currently indicated layer. The 'Scale' slider below the plan allows you to adjust the size of the plan displayed to best match the size and complexity of the structure you are building. Again, moving the slider left and right dynamically scales the plan above it. You can scale down to 1/4 original scale, and up to 2x original scale.