Cathode bypass capacitor

[Champ81]

I had a question regarding the cathode bypass capacitor.  Whether in preamp or power amp sections.  I know and have witnessed how it affects the tone of the amp and increase gain. From my current knowledge it compensates for the frequency loss due to the resistor allowing certain frequencies to pass through depending on the value.  My understanding is the cathode resistor value determines how hot or cold the tube is biased. So essentially the amount of electrons that emit from the cathode. Higher value resistors emit less and vice versa. The grid is sort of like a valve that controls how much of the electrons go from cathode to anode (the plate).   

If I got that part correct I don't understand how a cathode is associated with the frequencies it let's through.  Because isn't it the grid where the guitar signal goes into?

I cannot visually understand it. I have heard from others a DC current flows from the ground through the cathode resistor to the cathode of the tube. Bit how van current flow from ground because the ground there is no current? I know ground is all relative but this is circuit ground to the chassis.