What is the behavior of electrons in a highpass filter circuit and what is an equivalent closed loop circuit?
First note that, yes, electrons will move a little, but this is not something you should be caring about during typical circuit analysis. Instead you should be thinking about the movement of charge. If you do not understand this, it's crucial that you backtrack and study how current and charge carriers work.
You also need to understand how a basic voltage divider works in order to answer this:

What is the meaning of that ratio?

That ratio is a simple voltage divider, where the bottom element is a resistance and the top element is a reactance (X). If you do not understand voltage dividers, again, it's time to backtrack and develop a solid understanding before proceeding to AC analysis.
The simplest, most intuitive way to understand this circuit is that &quot;capacitors block DC current and pass AC current&quot;. Since the capacitor is in the path of the current to the output, it will only allow AC current. The resistor will shunt away some current. The size of the resistance compared to the capacitor's reactance will determine the frequency past which current will be blocked: the cutoff.
Do not introduce a resistive load until you understand the above. The resistive load will change the behaviour of the circuit, effectively reducing the total resistance and changing the cutoff frequency.