She talks to herself (and us). She jumps at the sound of owls and she talks about how well she's prepared the scene: she's drugged the guards and laid out the daggers nice and neatly for Macbeth, and there's a suggestion in this speech that she expects Macbeth to bungle it. Of the daggers, she says "he could not miss them," and of the act, she says she fears "the attempt and not the deed confound [them]," meaning she thinks they may be caught in the act.
The noise that startles her--the owl (fatal bellman) is ironic because the cry of the owl is supposed to signify an impending death. Given that she has planned the death of Duncan, her reference to being frightened by the owl is particularly powerful--it is as if she is already afraid of the nature of the act (although it takes her much longer to admit it). She actually reveals quite a bit of her weaknesses here: jumping at the sounds of night creatures, expecting things to go wrong, and not being able to actually stick the knife into Duncan because of her sentimental moment, all show that she is more of a plotter and a planner than a doer. Even as Macbeth acts, she lurks about thinking, and worrying about possible complications and consequences.
Lady Macbeth also takes a moment to explain why she didn't kill King Duncan herself. She says that Duncan looked like her father as he slept, which is why she couldn't do the job. Shakespeare here foreshadows Lady Macbeth's remorse and gives the first indication that she isn't as confident as she likes to pretend.
Lady Macbeth waits.
To be a bit more detailed, she waits and frets. Every sound she hears seems like a warning or alarm--some signal that things have gone wrong and/or that her husband has been caught. She jumps when the owl cries, for example.