SARAH: Fortune was sometimes represented as a strumpet — a harlot or loose woman.

RALPH: This is because one minute you would be in her good favor, and the next you find yourself in her disfavor.

SARAH: Gildenstern says that he and Rosencrantz are not super happy — they are not the button on her cap; but neither are they miserable, the soles of her shoes.

RALPH: Hamlet then makes a bawdy joke by saying that they must live about her waist, in the middle of her "favor" — the part of her body she offers as a treat.

SARAH: Gildenstern continues the joke with: "We are here privates".

RALPH: Meaning "We are just private citizens receiving average favor", but also, "We are in her 'private' parts."