what is diffrence between specular map and metalness ?

Hello Everyone,
I am learning pbr workflow but dont understand the diffrence between specular map and metalness .
as i know about albedo ,roughness ,normal,ao,etc but i dont understate difference between specular and metalness .please tell me in simple way.

thanks

Metal is for when a material is metal… specular is a “remnant” or alternative way for physically based renderer, and is replaced by roughness in UE4.

here is a lot of discussion about it: