What is meant by "nerves" in chess? It is said about chess players in general. For example, Wikipedia says about Ivanchuk:


  His inability to become World Champion despite his immense talent and longevity among the chess elite has been attributed to his admittedly poor nerves, which were exposed during the high-tension atmosphere of World Championship match-format tournaments


What does this "poor nerves" mean? Does this mean he cannot cope up with the stress?
Yes to both questions.

At the highest level of any sport/game/activity, there is an extreme amount of pressure.  Being in a foreign city, the quest to find the best move, the knowledge that most chess players will be going over you game, annotators criticizing your every move, and the added pressure of a ticking clock.  (Digital clocks don't tick, but most chess players prefer this reference.)  Professional chess players have been known to lose 20 pounds(9 kg) during a normal 2 week tournament, due to the stress of the situation.  Although I don't think they consider this, one misplay and they will be remembered like Bill Buckner.