What is weathering?

Weathering is a painting technique used by modellers to create the appearance of dirt and grime and even damage to make the model "whatever it may be" look aged and used.

For example: If you have just built and painted a model of a military Jeep, you may want to give it the appearance of having gone through a muddy field. This can be achieved by adding an earth colour to the walls of the tyres.

To get the correct realistic look I would suggest looking for pictures of what you are trying to recreate.

Either on line or in books also out in the real world. The possibilities are endless for creating battle damage rusty parts and general wear and tear.