What Does \mu Represent in Free Body Diagrams?

[moondawg]

Homework Statement

What does $$\mu$$ represent?

Homework Equations

f=$$\mu$$N

The Attempt at a Solution

I understand it is a coefficient but I don't understand what it stands for especially when being applied to Free Body Diagrams. Explain please?!

 

[pongo38]

Could be coefficient of friction, the ratio of frictional force (f) to normal reaction (N)?