Sabé is one of Queen Amidala's five handmaidens. However, Sabé's role as a handmaiden isn't simply to assist the Queen with her elaborate gowns and hairstyles, as one might think. Sabé is also highly trained to act as a bodyguard to Amidala, as well as her close friend and confidante.
Sabé is Amidala's most trusted and most important handmaiden. Because of her resemblance to Amidala, she can act as the ultimate bodyguard, and take on her role as Queen in times of danger. Beneath the white makeup and robes, it is near impossible to distinguish between the two of them.
However, just looking like the Queen is not enough. Sabé had to train with Amidala to learn how to speak and act regally, and emulate Amidala's speech patterns and behavior, so that she is indistinguishable from the real Queen. The two of them also developed a secret form of communication by the use of subtle signals, so that Sabé can still follow the Amidala's wishes when she is playing the Queen.
For instance, when Sabé must decide whether to remain on Naboo or go to Coruscant, she says, Either choice will present great danger.. to us all.., while glancing subtly toward Amidala. Amidala replies with We are brave, your highness.., meaning she wishes to go to Coruscant. While this may look like simple conversing between the Queen and her handmaidens, it is actually a highly refined form of communication.
Sabé's ultimate role as the decoy queen is to sacrifice her life for Amidala's. She accepts that fact and faces it bravely each time she dons the Queen's robes.