what exactly is shader?

Hello, everybody! I know this may sound strange and funny to you. However, I wonder what exactly is shader? What will it do to our assets? And how can we use shader? Thanks in advance!

A shader is a program which is passed to the Graphics Card for converting 3d model information and texture data into pictures you can see on your screen! Almost everything you see in real-time 3d applications comes from some kind of shader. You can use a shader in Unity by assigning a material to your object in the Mesh Renderer inspector.

Unity comes with a large selection of shaders for all sorts of different purposes. They are described in detail here. If you need special functionality, you can define your own shader in the CG shader language- but for most cases the Unity inbuilt shaders are more than enough!