Mesh Filter vs Mesh Renderer

Mesh filter is the form of an object on the screen.
While Mesh Renderer is the way it will look like on the screen.

Examples:

here is the mesh (filter) of a cube object, this is the form of how it will be displayed on the scene

in the above example you can see that the mesh renderer (how it will look like) material is Default-Material and you can change it to any material you want.