Modelling is broadly defined as creating something, or part of something, that performs as closely to the final product as possible. This allows designers to test how well it works and also to create many cheap variations as quickly as possible so that many alternatives can be developed, improved and approved or rejected.
Any modelling material can be chosen as long as its performance characteristics align closely with the intended materials.
Modelling can also be carried out using CAD.
A very common form of modelling is done on spreadsheets to model volume and weight of materials, costs and numerical projections, for markets, environmental damage etc.
There are also three main types of 3D physical modelling to consider as you develop work:
Sheet
Frame
Moulding
(CAD has it's merits and advantages, but why do you think that all car designs go through CAD modelling and are then ALWAYS made full size, by hand by a highly skilled team of sculptors in clay before the concept gets its final approval for manufacture? It is a much more appropriate modelling technique, but why?)