Usually it means "I'm tired", but in some contexts it can be talking about a longer term state of life. For example, I might say, "He used to be the best lawyer in town but he's faded now." The meaning is still "tired" but less literally. It means that his best is now in the past. In this context, the metaphor is a "faded star" that no longer shines brightly. A "faded rock star" is a common example.