public Material[] sharedMaterials { get; set; }

This is an array of all materials used by the renderer. Unity supports multiple objects using a single material; in this case contains all the materials used for this. Renderer.sharedMaterial and Renderer.material properties return the first used material if there is more than one.

Modifying any material in will change the appearance of all objects using this material, and change material settings that are stored in the project too.

It is not recommended to modify materials returned by sharedMaterials. If you want to modify the material of a renderer use Renderer.material instead.

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.

Additional Resources: Renderer.material, Renderer.sharedMaterial properties.