XNOR (Exclusive NOR) is the opposite of XOR (Exclusive OR)
A way to remember XOR is "must have one or the other but not both" XNOR is the opposite so it's "must have both or none" so both inputs must be 1 or 0.
XNOR or XOR is very common in homes with staircases so that you can turn on and off the light in the staircase regardless of which floor you are on.
If you google staircase switches, you will be told that they mostly use XOR but according to the wiring diagrams they use XNOR.
I was gonna say lol that's a fancy way of describing three-ways
Fun fact, you can put as many 4-ways in between those 3-ways as you want, or as many as your electrician will agree to anyway. You can switch the same light from a dozen spots if you really want to.
I understand it now, but I'll have it forgotten in five minutes. I'm a software engineer, this here is dark magic to me. Well, not this, but more complicated stuff. I envy you all for your big, juicy brains.