A three-dimensional object is one that exists in three planes, which typically means it has width, length and depth. The three most common directional dimensions are x, y and z.
An example of a three-dimensional object is a cube or pyramid. Both of these objects consist of multiple two-dimensional shapes. Because the shapes are laid out so that they share only one dimension with each other, the combined shape gains an extra dimension to become three-dimensional. People see the world through three dimensions of space because all visible objects are three-dimensional. The only two-dimensional objects are projections created on surfaces.