Recall the last letter of your first name. It's an easy task, but it highlights the difference between unconscious, conscious, and locus of attention. When you began the first sentence of this paragraph, your locus of attention was on the words of the sentence, as it is now. Humans have only one locus of attention, and are able to think of only one thought at a time. When you read the first sentence, it made you bring the last letter of your first name from your unconscious, to your conscious, thus causing your locus of attention to change. Notice how you had to stop reading until you thought of the letter, then you shifted your attention back to the text.
Your locus of attention is important in the design of computer interfaces. An interface which requires constant shifting of your locus of attention is frustrating. Changing your locus of attention slows down tasks. While learning a new interface, some additional time is required to become familiar. But over time, a well designed interface will not distract a user more than necesary from the task the user wishes to complete.
Humans naturally form habits. Habits allow humans to move certain actions from the conscious to the unconscious. Interfaces which exploit this fact are successful. For exmaple, both Windows and the Macintosh offer consistent buttons for closing windows. They are always placed at the same location in every window. As a user, you don't have to constantly recall how to close a window, you can rely on your unconscious to do the work.