The meaning of a white feather can be anything from literal to legendary. Different kinds and different coloured feathers represent different things in different cultures. Even the meaning of a white feather means one thing in some cultures and completely the opposite in others. The literal explanation is the only one unequivocal in meaning. Literally, the meaning of a white feather is that it came from a bird with white feathers. The rest of the meanings are much more interesting and steeped in lore.
To Native Americans a white feather represents purity and wholesomeness of heart. A Native American Indian given the name Whitefeather is translated to mean the bravest of the brave.
Oddly enough, there is a connection between white feathers and ravens, which are black. Ancient Greek legend holds that ravens were once all white and cawing all the time. Because the raven couldn't stop its incessant yakking it was believed to be sharing secrets repeatedly. As the story goes, Apollo grew weary of the bird's penchant for betrayal by talking too much so he punished the raven by turning its bright white feathers black. There is a corollary that maintains the owl replaced the raven at the side of Athena but was anthropomorphically given a gift, not just for gab but wisdom. Thus the phrase "a wise old owl."
This story originated in England near the turn of the 18th century when cockfighting was a popular form of gambling-sport. Some of the game birds tossed into the fighting ring had white tail feathers and, not anxious to fight, would turn away from their competitors, showing their white feathers. It is one possible origin of the phrase "to turn tail." Showing white feathers came to signify cowardice among humans, a term used in many contexts, from literary to war. The phrase "Turning tail" became particularly popular in English usage throughout the 19th Century.
The White Feather Campaign
In 1917 and 1918, England instituted conscription to shore their military ranks in anticipation of entering WWI. Men who weren't drafted or didn't volunteer were looked down upon, mostly by the women who kept the home fires burning. A non-uniformed man's presence in England was viewed as a sign of cowardice or womanliness. Women would present a white feather to Englishmen in civilian attire as a token symbolising their scorn toward the man, saying in effect that they weren't real men because they weren't fighting in defence of the country. On some levels, it was an attempt to encourage civilian men to enlist.
The Boer War
This story is based on British folklore and relates to the The Boer War, which preceded WWI. The story is told with great specificity so there may be at least a grain of truth to it. Retired Admiral Charles Penrose Fitzgerald is said to have organised 30 women in Folkestone in 1914 to hand out white feathers to any men that they saw not in uniform as a symbol of cowardice and as a means to encourage enlistment. This was said to have been reported in the press and the practice spread. The women came to be known as members of the White Feather Society. If true, the custom may have been the likely predecessor to the White Feather Campaign.