Lady Macbeth is referring here to the guards who protect King Duncan. Part of the fiendish plot to kill the king in order for her husband, Macbeth to succeed to the throne involved drugging the king's guards. It is Lady Macbeth who conceives the plan to ‘drug their [the guards] possets.’
The whole household of Macbeth has been celebrating the arrival of King Duncan. His visit was to give thanks to Macbeth for his bravery in battle in defeating the Norwegian forces. Lady Macbeth has convinced Macbeth that killing the king will advance the prophesy told by the witches that he should be king. Macbeth is not so sure of the plan, but Lady Macbeth convinces him of the necessity to kill the king.
As the evening progresses she becomes more determined in her plan. The adrenaline (and alcohol) which has fuelled the evening has given her courage to carry out the plan, hence –
That which hath made them [the guards – and possibly the other guests] drunk hath made me bold.
The above quote ties in with Lady Macbeth's earlier statement in Act l, Scene 7, as a means to persuade her husband to continue with their evil plot to assassinate the king. He believes their plan might fail. After admonishing him for being a coward, she says the following:
His two chamberlains
Will I with wine and wassail so convince
That memory, the warder of the brain,
Shall be a fume, and the receipt of reason
A limbeck only: when in swinish sleep
Their drenched natures lie as in a death,
What cannot you and I perform upon
The unguarded Duncan? what not put upon
His spongy officers, who shall bear the guilt
Of our great quell?
This scene indicates how devilishly ruthless and sly Lady Macbeth is at this point. She will deliberately ply everyone with alcohol to ensure their "swinish sleep." They will sleep "as in death," and will be totally unaware of what is happening. Once she and her husband have committed their evil, it will be easy to implicate Duncan's drunk guards, who will, obviously, not know anything. Macbeth is impressed by her cunning and agrees to follow through.
Lady Macbeth utters the quote in Act ll, Scene 2. She has succeeded in making King Duncan's chamberlains drunk by the above means. She has also drugged their drinks, ensuring they will be dead asleep. Since the guards would be completely incapacitated, neither she nor her husband would be challenged and would, therefore, be free to commit their evil.
Therefore, it is obvious that Lady Macbeth is emboldened by this situation. Her courage is derived from the fact that the guards will not be able to defend their king at all. She means that the "potions" and "wassails" she gave them to make them drunk has given her the courage to continue with their pernicious plot.
As it is, she and her husband's plot is successful -- Duncan is murdered and Macbeth kills the guards, supposedly in an act of instinctive retaliation. It is ironic that Lady Macbeth is so merciless here, for later in the play she is so overwhelmed by guilt and remorse for what they have done that she starts walking in her sleep and eventually commits suicide.