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).
In mathematics (maths), length, width, and height are given letters (variables) to make them easier to write, or if a value is not known. X is width, Y is height, and Z is length. Another common 3D coordinate system is the spherical coordinate system.
The mathematical study of shapes and dimensions is called geometry. Geometry can be used to measure how tall a mountain is.