I'm kinda new to digital electronics.My question is that:

What does this output characteristics mean(in the attachment)?
The schematic is attached too.

Any help is much appreciated.

Thank you,
Krisztián

 

Looks like the output voltage rise from sink current changes.

 

Yes. The inputs are connected high so the output would be low. The graph shows that with zero output current the voltage would be about 0.2V and if the output is sinking current the voltage rises to about 0.5V when the sink current is 40mA.