The source impedance governs the "current drive" of the source (i.e. how much current can be drawn from the source before the input to a circuit starts dropping the output voltage of the signal source). Example, if you have a signal source that is capable of sourcing 1mA of current at a 1V output, your source impedance would be 1K. If it was capable of sourcing 10uA @ 1V, the source impedance would be 100K. The less current it is capable of sourcing, the higher the source impedance.
The load impedance is either the impedance of the input to the device or the impedance of the load the device is driving on its output. In the case of an audio power amplifier, the latter of these would be the speaker impedance.