The Concept of CollisionLayer and CollisionMask about Physics

Hi , Dear everyone!
After I read some document , and do something follow example code.
My code is worked.
But I can’t understand layer and mask , what is that ?
I have no experience about physics, thanks .

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Hi, Please use the search function, the forums are full of topics on this argument.

Anyway, this explains really well the concept of layers and masks.