They're speaking of the Thevenin equivalent source and load:
The source voltage and resistance, and the load voltage and resistance, are in series. You can rearrange this so there is one voltage (the source and load voltages subtract), and two resistors in series vertically, as a resistor divider.
Note that it's symmetrical too, which isn't always important (most loads are understood to be unpowered, so that VL = 0). But this becomes relevant for bidirectional circuits (many communications channels), and at RF, where it's an important analytical tool, used to abstract away a load's complicated behavior (i.e., the load reflects power back towards the source), by leaving a placeholder to make that connection in a separate step.
Tim