You use the material editor whenever you need to design the colour, texture, transparency, and other visual properties of an object's appearance.

Every visible surface has a material. You can make them shiny, dull, transparent, and opaque. You can texture a surface with an image used as its material's colour, or to lighten or darken its basic colour. You can also use a texture to control the transparency.

The lighting will also affect the appearance of objects in your scene.

Materials have a number of components.  The main ones are:

Textures

While you can select basic textures from the materials dialogue, clicking the textures button will give you access to advanced textures.

There is a lot more information in the User Manual under Materials.