High Pass vs Low Pass simple Circuit (RC vs CR)
For years I tutored the very first electrical engineering course at my uni, so this comes up a lot - don't worry :)

The best way to understand this is to think about all 4 cases:


An AC (or, you could think about it as a "high frequency") signal through the low-pass filter
An AC (or, you could think about it as a "high frequency") signal through the high-pass filter
An DC (or, you could think about it as a "low frequency") signal through the low-pass filter
An DC (or, you could think about it as a "low frequency") signal through the low-high filter


Key thing is to substitute in what the impedance of the capacitor would be (the resistor has the same impedance as it does resistance, so that doesn't matter). For a capacitor:


High-frequency would mean the capacitor has a very low impedance (i.e. high-frequency can pass through it)
Low-frequency would mean the capacitor has a very high impedance (i.e. just like the symbol for it - two open plates - the low-f or DC can't pass through easily)


With these things in mind, let's think about the path the current takes in all 4 cases:



Note: (and for those who are going to call me out) the "output" will actually be some resistor it is going towards, not an open-circut like we always draw when designing filters.

You can see here for the 4 cases:


For AC, the low-pass filter blocks it from reaching the output as it goes to ground first, because the capacitor offers a low-impedance path to ground (remember: all signals, DC and AC, want to go to ground. That's their end goal)
For AC in the high-pass, it passes through as the capacitor is an easy path to get through, and the resistor is just there doing it's thing - not offering any easy path to ground
For DC, the low-pass is just like having a resistor in the way, as it can't pass through the capacitor easily and so the path to ground is not an easy option
For DC, the high-pass blocks it almost before it even starts flowing, because the capacitor is present and as I mentioned earlier: for DC/low-frequency signals, the capacitor is just like it's symbol - it blocks the signal, and presents a high-impedance path.


I hope this helps. Happy to clarify if my explanation isn't clear enough :)