Output Voltage Swing vs. Load Resistance of an Op Amp
When load resistance decreases, current increases (assuming voltage is fixed). By the looks of it, the opamp you are dealing with can only provide 25mA of current.

In order to ensure that the load doesn't try and source or sink more than 25mA, the graph you provided just shows what the suitable voltage levels are for your resistance.

For instance, lets say your load is 100 ohms. The maximum current you can draw is 25mA.
So

$$ V = IR $$
$$ V = 25mA * 100 ohms $$
$$ V = 2.5V $$

Which what you're graph is showing. 

So instead of doing this calculation for every single load, they have done it for you. 

All this plot is telling you is not to go over 25mA.

Added
Found this in the datasheet