3D (or 3-D) means three-dimensional, or having three dimensions. For example, a box is three-dimensional; it is solid, and not thin like a piece of paper. It has volume, a top and bottom, left and right (sides), as well as a front and back. You can turn the box around to view it from another side (called a face, surface or simply side).
A dimension can be measured in inches, microns, miles, kilometers, or any other unit of measurement.
The three dimensions are often called length (or depth), width (or breadth), and height. Some purposes use other words.
3D graphics are used to make video games or animated movies. Many calculations are needed to make pictures seem three-dimensional on a screen. Modern computers usually have a GPU (Graphics Processing Unit) to handle these calculations.
Binocular vision helps people to see the third dimension easily.
In the Cartesian coordinate system, length, width, and height are given in letters (variables) to make them easier to write, or if a value is not known. Often X is width, Y is height, and Z is length. Other 3D coordinate systems such as spherical coordinates use both linear and angular measurements.
The mathematical study of 3D shapes and dimensions is called solid geometry.
Some people think of time as if it were a fourth dimension. This is called space-time theory. In this idea, there are three dimensions of space, along with the dimension of time.