There are numerous references to clothing in the play. I assume that you mean Act II, scene 4 which occurs after the murder of King Duncan. The robes referred to by Macduff are the robes of office, in this case the king's robes. Interesting enough, when Macbeth is greeted in Act I, scene 4 by Ross, as the Thane of Cawdor, he asks,"...Why do you dress me /In borrowed robes?"
There is a certain irony in Act II, scene 4 when Macduff tells his kinsman, Ross, "Lest our old robes (Duncan) sit easier than our new (Macbeth)."
Is Macduff being cautious or does he already suspect that all is not what it seems to be?
The clothing motif is found in many scenes throuhout the play Macbeth.
In Act II scene iv the clothing motif occurs in the following lines:
"Well may you see things well done there; adieu
Lest our old robes sit easier than our new."
After king Duncan's murder,these lines were spoken by Macduff, a general in the king's army to Ross, a nobleman of Scotland when the latter said he was going to Scone for Macbeth's coronation.
Macduff hopes that Ross would see things done well at Scone. He then bids farewell before they find that the new state of affairs is not so comfortable as their previous condition.
In these lines we first get the glimpse of Macduff's integrty.