The line reads,
Well, may you see things well done there. Adieu,
Lest our old robes sit easier than our new!
This especially important line occurs at the very end of Act 2, scene 4. It is spoken by Macduff to Ross. In this conversation, we learn that Macbeth is soon to be crowned king of Scotland at Scone. Macduff has decided not to see Macbeth crowned king, but instead to return to his home. He asks Ross to see the ceremony is carried out properly, and then he uses the clothing metaphor to express his concern about the changes in leadership that Scotland is undergoing.
Macduff is worried that the new leadership (new robes) will not be as good as the former leadership ("old robes"). From this seemingly innocent remark, we see that Macduff doubts Macbeth's ability to rule Scotland effectively and that he may suspect Macbeth as having a role in Duncan's assassination. Before this line, Macduff has answered each of Ross's questions literally and objectively. He tells Ross that the guards killed Duncan, that the kings' sons are suspected of hiring the guards to do the deed, that Macbeth has already been named king and is going to Scone to be crowned. However, the couplet you refer to seems to stand out, because for the first time Macduff expresses concern for the future of Scotland under Macbeth's rule, and he uses figurative language to express this concern.
This line is part of the clothing motif that runs throughout the play. For instance, Macbeth asked Ross earlier
Why do you dress me in borrowed robes?
when Ross told him that he had been given the new title of Thane of Cawdor. Macbeth wondered why he had been given this title when the current Thane of Cawdor was still alive. Macbeth did not know that the former was soon to be executed as a traitor. Macbeth, in fact, did borrow the robes of this traitor in his assassination of Duncan. Clothing is a metaphor for roles, and it is an especially appropriate symbol since clothes, like roles, may or may not fit. Macbeth could wear the robes of a traitor; can he wear the robes of a king?