From Mr. Welles himself:
The most basic of all ideas was that of a search for the true significance of the man’s apparently meaningless dying words. Kane was raised without a family. He was snatched from his mother’s arms in early childhood. His parents were a bank. From the point of view of the psychologist, my character had never made what is known as “transference” from his mother. Hence his failure with his wives. In making this clear during the course of the picture, it was my attempt to lead the thoughts of my audience closer and closer to the solution of the enigma of his dying words. These were “Rosebud.” The device of the picture calls for a newspaperman (who didn’t know Kane) to interview people who knew him very well. None had ever heard of “Rosebud.” Actually, as it turns out, “Rosebud” is the trade name of a cheap little sled on which Kane was playing on the day he was taken away from his home and his mother. In his subconscious it represented the simplicity, the comfort, above all the lack of responsibility in his home, and also it stood for his mother’s love which Kane never lost.
source (the rest of the article is an interesting read in itself)
