What is output impedance of a pin?
An output pin looks something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

Vout is an ideal voltage source, R is the output resistance of the source and C is the output capacitance of the pin.

Let's ignore the cap for now.
An ideal output pin would include the voltage source only, but unfortunately we're unable to build voltage sources with zero internal resistance, so when you actually make a voltage source it will have a series resistor which value is the "internal impedance" of the source. If your voltage source is used to drive a pin then that same R is also called the "pin output resistance", i.e. the resistance that you measure between out and reference with all the sources turned off. The output resistance is an important parameter because it limits the load resistance that you can connect to the pin: if you have some \$1k\Omega\$ output resistance and you want do drive a \$1\Omega\$ load you will have a bad time, since the output voltage will almost not vary because of the voltage divider you are de facto building.

So what's this C about? That's another parasitic component, it comes from traces on the Silicon die, parasitic transistors capacitance and so on. The capacitor is important because it limits the speed of your pin, so it's a dynamic parameter, while the output resistance is a static one. As you can see the RC components form a first order low pass filter: the higher the C, the lower the corner frequency \$f_C=\dfrac{1}{2\pi R C}\$, the lower the maximum speed you can toggle your pin while keeping a clean wave.

I added the C part since you speak of the output impedance, that should include the capacitor. What you find in datasheets is usually the output resistance (\$\Omega\$) and the output capacitance (F) as separate values.