In Macbeth, what does Lady Macbeth mean by saying that their service, even if doubly redoubled, would be a poor return to Duncan?

All our service
In every point twice done and then done double
Were poor and single business to contend
Against those honours deep and broad wherewith
Your majesty loads our house...

All Lady M is saying in this extract from Act 1, Scene 6, is, even if every act ("point") of serving Duncan that the Macbeths do were done twice, and each of those done twice, it still would be only a "single" (i.e. not a double, or quadruple) service compared with how much service Duncan is doing them. And what service is Duncan doing them? - he is bringing his "honours... deep and broad" to their house, and thereby making them honourable.

Hope it helps!