A sanskrit term whose meaning approximates to "auspicious vision", darshan is most often used to refer to the sighting of an enlightened Master or Divine being. Thus, one can receive darshan from one's guru, or one can receive darshan of, for example, Krishna or Rama in a vision or a dream.
Darshan is extremely important in the Hindu esoteric traditions as described in the Vedas and the Upanishads - the mere glimpse of a realized being is enough, supposedly, to burn away lifetimes of accumulated karma in an instant, and sadhus and Yogic ascetics will practice their disciplines and recite their mantras for years or even decades to obtain just one such vision or glimpse. The followers of famous gurus such as Bhagavan Nityananda or Ramana Maharshi would make long, arduous pilgrimages to see them, bearing valuable gifts, and queue for hours outside their room, just to catch sight of them once or hear them speak.
The importance of darshan resides in the fact that the sight of the enlightened guru or Divine vision communicates the realization of the Divine directly, bypassing conscious thought, emotion and spiritual practice. In other words, though the conscious mind may not realize it, one is gazing upon a manifestation of the Divine, the very essence which the Hindu spiritual practitioner has dedicated themselves to realize, and the sight of that manifestation remains in the mind forever as a guide.
Darshan can also be used to refer to the simple sighting of a picture or iconic representation of the Divine, in any form. This is a less powerful meaning, however, because one of the essential qualities of the true (Divine) darshan is that not only does one see, but one is seen by the living Divine force.
As with any belief, some people take the concept of darshan to extremes. I have seen otherwise balanced and self aware people fighting over the front seats at a formal darshan occasion, or waiting outside their guru's bath house for hours on end in the hope of catching a glimpse of him. I guess if you believed your salvation or enlightenment could be attained by such a glimpse, then anything you did to get it would be worthwhile.