"The apron is perhaps the most symbolically important emblem found in speculative Masonry. It is made of pure white lambskin and is worn by every Mason to all Lodge events. The apron is the first symbol explained to the Entered Apprentice (1st degree, Blue Lodge) and the first tangible evidence that he possesses of his admission into the fraternity. What the Mason probably doesn't know at the time of his initiation is that the apron is also worn by temple Mormons and active practitioners of Wicca (witchcraft)."
A more detailed history of the apron and its symbolism is available here.