Just prior to this statement, the king, Duncan, has thanked Lady Macbeth for the pains she has taken to prepare her home for his coming; it would have been a substantial undertaking to host a king with all his attendants and train, especially on such short notice as she had. Remember, she only received word that Macbeth and Duncan were on their way a few moments before Macbeth actually arrived home in scene 5. After the king thanks her, she says,

All our service,
In every point twice done and then done double,
Were poor and single business to contend
Against those honors deep and broad wherewith
Your Majesty loads our house. (1.6.18-22)

She means that they could not possibly go to too much trouble for Duncan, especially after all of the honors upon which he has bestowed them of late. First, after he had the Thane of Cawdor executed for treason, and he awarded that title to Macbeth, giving him more power, prestige, and, probably, more land to boot. Second, he's honoring the Macbeths all the more by traveling to and staying at their home. Therefore, Lady Macbeth implies that his thanks for their hospitality are unnecessary as a result of everything he has done for them.

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!