Situation in which a player scores by inadvertently hitting the ball in such a way that it touches the upper cord of the net and rolls over to the other side; the player is said to have “gotten (caught) a dead net (dead net cord)” and considered to be lucky. (Wikipedia)

I saw this clip of Mensik celebrating after getting a point on Djokovic. Novak seemed to be unhappy about it. Apparently this is what you call a net cord point, and players usually apologize after scoring one.