mfcc - what is the mel scale?

I am not sure I understand what the Mel Scale is.

Googling doesn't give me various answers. I seem to be getting the same response again and again.

Which would be something like:

"The mel scale reflects how people hear musical tone"

First of all, does the concert pitch has a specific frequency? So why do we even have a scale stating how do people hear it?

I am currently trying to understand how MFCC uses them, and it seems like it maps the frequency to the Mel scale, or there are some filter banks which are applied on the given frames