I assume you are asking for an explanation of the sentence, rather than a grammatical breakdown of each word. Please let me know if this is not correct.
This quote, from Edgar Allan Poe's short story, "The Cask of Amontillado," refers to Fortunato's final steps into the catacombs of the Montressors.
"In an instant he had reached the extremity of the niche, and finding his progress arrested by the rock, stood stupidly bewildered."
Fortunato has reached the end of the line. The "extremity" is the farthest distance; the "niche" is the small room or alcove (cubbyhole, corner). His progress is halted by the cavern wall, and he stands staring in wonderment. Fortunato has not yet figured out Montressor's true plan, but he will understand shortly.