A two-dimensional object is an object that only exists in two dimensions, most commonly width and length. A three-dimensional object typically has width, length and also depth (or height).
Everything that people see is in three dimensions. However, two-dimensional objects can be created by projecting a three-dimensional object on a flat surface, like a computer screen or piece of paper. There are many different two-dimensional shapes that can be drawn, which are called polygons. These are related to other three-dimensional shapes. For example, a square is the two-dimensional representative of a cube, and a triangle turns into a triangular prism if a height dimension is added.