A guideon is a sort of flag. It has silver rings on the lance poles, names of places engraved on them. These days every company in the army has one, but in the old days only cavalry had guideons. Noah Wellman tells about A troop, 7th cavalry. He tells of their guideon, red and white.
Fiddler's dream is where good cavalry men go when they die. It's something between heaven and hell, something just for the good cavalry men to work towards as their final ultimate reward for their dedication. Noah Wellman can't get into fiddler's dream. They won't let him in because he lost A troop's guideon. Noah wanders around outside, peering in, envying the people inside fiddler's dream. Now he tells the story of the guideon, and of how he lost it.
A guideon can give a soldier an identity. It can be a more important thing to fight for than the country, because it's more personal. Even the name itself implies that's it's supposed to guide onward. The person assigned to carry it has an immense responsibility, and must bear the consequences no matter the circumstances that fate may prescribe.
"Men always have to have somethin' to follow, to believe in -- a guideon."