The meaning of the word metal

Kindly explain what does the word metal mean here?

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I am not even sure how to make a research. I guess it's some kind of metaphor here.

Computers are made of metal. From the browser down to the metal refers to the software and the hardware involved.

From a post on gamedev.net:

there was a time when "down to the metal" meant basically writing code for the raw hardware (with no OS or device drivers involved), and often with stuff written in a fair bit of assembler as well.

"Metal" refers to the physical hardware, trying to cut through the driver or operating system levels, i.e. low level CPU, graphics programming, real-time embedded systems. Some call themselves "down-to-the-metal C++ programmers".

It might be related to the expression: "pedal to the metal", where (gas) pedal is fully depressed to the car floor, providing as much speed as possible, which is one of the purposes of "to the metal" programming. Additional shades are given in Close To The Metal, referring to code limited to some hardware or security issues..