For the purposes of the movie "Citizen Kane", "Rosebud" was the brand name painted on the sled he had as a small child as seen fleetingly in the flashback of him as a kid and then in the final furnace scene. As you may recall, the scene of him dying in bed has him dropping a snow globe as he utters the famous line. It may have been what made him think back to his time of innocence in his youth with that sled, probably one of his most cherished possession at the time since his family was so poor. It worked for the plot that that was what it meant and why no one in the movie could figure it out. It was a beautiful, poetic payoff.
Forum about the meaning of Rosebud in Orson Welles' "Citizen Kane"