3D modelling is the creation of a subject on a computer. It is the process of developing a mathematical representation of that subject in three dimensions using 3D modelling software such as 3Ds Max and Solidworks. 3D modelling is the standard within a computer-aided design. A 3D model can be used for almost anything and is essentially how most things begin life on the journey to production.
3D models can contain a large set of information such as dimensions, material, weight, the center of gravity, and many more depending on the project requirements. You could think of it as a blank piece of paper and the computer is the pen.