The metallic defines whether the material is metallic or not.

With metallic=0, the material is non-metal and the metallic map is black.

Metallic = 1, the material is metal and the metallic map is white.

During the actual operation, we have to determine whether the material we want to make is metal or non-metal, and pick a value between 0 or 1. For metal, the metallic value is set to 1. For non-metal, the metallic is set to 0.

In the following figure, for example, when the base color is yellow, metallic=1, the material looks like gold; metallic=0, the material looks like yellow plastic.

How does it work when the Metallic value is between 0 and 1?
Since most materials are either metal or non-metal, why is the metallicity control a continuous slider of 0-1? Can't you make a switch option? This is a question many users will have. It is also a topic worthy of explanation.
The metallicity of pure metal materials should indeed be 1. However, if the metal is covered by rust, pollution and dust, in this area, it is essentially a mixture of metal and non-metal, then its macro metallicity will be less than 1 or even close to 0. Metals covered by dust and oil stains or oxidized and rusted metals are generally treated as non-metallic, and the metallicity of rust will be set to 0 or very close to 0.
The metallicity of some semiconductor materials, such as silicon and germanium, is also between 0-1.
In addition, in the process of making materials, we often encounter not only single materials, but also mixed materials. For example: locally rusty metal. In this case, we need to define the metal rust value between 0 ~ 1, which is very useful in this case.