output resistance
Output resistance measures how stiff or floppy the output of a stage is, how much its output voltage sags when you draw current from it. It is the effective resistance you see looking back into the output terminal, which is precisely the Thevenin resistance of that stage. A low output resistance is like a big rooftop water tank with a fat pipe: open many taps and the pressure barely drops. A high output resistance is a thin straw that gives up its pressure the moment you sip.
You can measure it from how much the output droops under load. If drawing an extra 10 mA makes the output fall by 0.1 V, the output resistance is Rout = 0.1 / 0.010 = 10 ohm. An ideal voltage source would have zero output resistance, holding its voltage no matter the load. A well-designed amplifier or regulator output gets close, with output resistances of a fraction of an ohm, while a raw voltage divider can have an output resistance of thousands of ohms.
Output resistance is the other half of the stage-connecting conversation, paired with the next stage's input resistance. For clean voltage transfer between stages, you want a low output resistance feeding a high input resistance, ideally the output resistance at least ten times smaller than the load it drives. The honest exception is radio and other matched-power situations, where you deliberately make output and load resistance equal instead, accepting the efficiency cost to maximize power into the load.
An emitter follower might have an output resistance of about 25 ohm, so it can drive a 1 kohm load with almost no droop. The same signal taken straight from a 10 kohm divider tap would sag badly under that load.
Low output resistance feeding high input resistance transfers voltage between stages cleanly.
For voltage transfer between stages, make output resistance low and the next input resistance high. For matched-power RF you instead set them equal, trading efficiency for maximum delivered power.