The word "afeard" means "afraid." Lady Macbeth, in this scene, is in the throes of a mental breakdown and is reliving the act of talking her husband into murdering their king. She is essentially re-stating her basic words to Macbeth, given in 1.7, when she was working very hard at convincing him to murder Duncan.
In a paraphrase of her re-statement, in 5.1 she is saying: You are a soldier, and afraid? It doesn't matter who knows what we've done, we're powerful enough to avoid being held accountable.
lord, fie! a soldier, and afeard? What need we
fear who knows it, when none can call our power to
Lady Macbeth is guiltily reliving her crime. She is confessing her part in Duncan's murder. "Afeared" is how she helped persuade her husband to murder him. She is saying that a brave soldier should not fear it. There would be no one to prove anything.
This scene deals with the descent into madness of Lady Macbeth. Her servant summoned the doctor to help her, but the doctor realizes that she suffers from guilt. Lady Macbeth is observed repeatedly pacing, attempting to wash invisible blood from her hands, and lying back down again, in a repetitive cycle.