This is an array of all materials used by the renderer. Unity supports a single object
using multiple materials; in this case
materials contains all the materials.
sharedMaterial and material properties return the first used material if
there is more than one.
Note that like all arrays returned by Unity, this returns a copy of materials array. If you want to change some materials in it, get the value, change an entry and set materials back.
This function automatically instantiates the materials and makes them unique to this renderer. It is your responsibility to destroy the materials when the game object is being destroyed. Resources.UnloadUnusedAssets also destroys the materials but it is usually only called when loading a new level.