To me it means:
The sense of finger to sound connection - the smoothness of the tonal transition in response to velocity when compared to a sampled piano with 12-16 samples spread across the dynamic range, the interaction between sustained notes, the ability to respond to a full range of pedal motion.....just the general sense of control one feels.

There are sampled pianos that sound wonderful, certainly better than Pianoteq, when you hear some notes in isolation, or even when you hear someone play them well, but just don't provide you with the same sense of connectedness when you sit down to play.