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
To stand on @hotpaw2's answers, think of Mel as one kind of pyscho-acoustic scale, derived from a set of experiments on human subjects, others are Bark &amp; ERB

Why have such a scale? 


Imagine a 100 Hz sine wave playing in your head, .... wait for it to stick ...okay... Good.
Imagine now, a 200 Hz sine wave playing ... marinate in it. Good work.
Now compare the two, they have some "perceptual distance". Perceptual only because we're using your soft-grey matter as a measuring instrument.
Finally, if you were you repeat this experiment with 1100 Hz &amp; 1200 Hz, and again with 10,100Hz and 10,200Hz, your opinion of this percpetual distance might diminish, i.e. 100 &amp; 200 Hz can sound "farther apart" than do 10,100 &amp; 10,200Hz, even though in all cases the differences are equivalent to 100Hz.


These pyschoacoustic scales try to capture these distances from low (20Hz) to high frequency (20kHz).