For a computer interface full-duplex means it has as many channels going in as out.
So if a mixer is not full duplex, it might have 16 channels but when you record into your DAW it will only take the stereo L/R mix (2 channels) 16/2
If it is full-duplex you can record each of the 16 channels to a unique track in the DAW and then mix them. 16/16
It basically determines whether you have a mixer or a multi-track recording device and its almost always impossible to tell by looking at a board - you usually have to check the tech specs.
Maybe there is another more common term for it but hat is how I have seen it described on most manufacturers documentation