Lady Macbeth is comparing the Macbeths’ relationship with King Duncan to that of a peasant farmer with his landlord in a feudal society.  The only asset the farmer has is himself since the land belongs to the landlord. The land is on loan, “in count”,  and the farmer periodically makes an “audit” and pays his rent. Lady Macbeth is saying that since she and her husband are servants of the king, all of their possessions are on loan from King Duncan, and they are prepared to give them back at any time.