What is Source Impedance?
The device that supplies a voltage signal or a dc power voltage is the source. The source impedance is the internal resistor in series with that otherwise perfect source voltage.

A battery for instance may have a source impedance of about 0.1 ohms. See this link to Duracell's website and click on the pdf for an AA battery: -



It's the same for linear voltage regulators, microphones and anything that can generate an AC or DC voltage - there will be some internal resistance that may have to be taken into account when the device is connected to a load.