What does it mean by input and output impedance in a signal generator?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. The internal layout of the output voltage measurement instrument and the output impedance, RS.


  The impedance of the output is always 50Ω. For a load impedance of 50Ω, the displayed voltage is half the output voltage to reflect the voltage seen at the load.


I suspect that they are indicating that the setup is as shown in Figure 1. The generator has a 50 &Omega; source impedance and is designed to drive a 50 &Omega; load. In that situation the voltage at the load will be half that of the internal signal source due to the 2:1 voltage divider. The voltmeter has been calibrated to read half the actual internal voltage.



From the comments:


  So if I send a sine wave with Vpk at 1 V, the actual output from this will be a Vpk of 0.5 V?


Try it. 


Set the frequency down low enough so that it's in spec for  your multimeter's AC True RMS voltage range - typically 50 to 400 Hz. 
Measure the output voltage on a 50 Ω load. 
Record the built-in meter's voltage. 
Now see if you can reconcile the two. The built-in meter may be using pk-pk which will be \$ 2 \sqrt 2 \$ times the RMS value. 
Now measure the open-circuit voltage. It should be double reading you got in (3) above.