The term used by Macbeth literally means that he understands that Macbeth needs help and also foreshadows his own doom.
Step-by-step explanation:
Macbeth and Macduff both know of the prophecy and Macduff trusts his friend to make the right decision and to not fall for his ambition when he bids him farewell.
Macbeth on the other hand has already fallen and planning the murder of Macduff.
So when Macduff says goodbye by saying Adieu for Macbeth it would be more akin to saying that he is going to god.
Macduff means that he wishes Macbeth would remember God.