When you’re creating a site, your job is to get rid of the question marks. Users look at a page that doesn’t make them think, thus there are the thought balloons over their head with positive signs. When they’re looking at a page that makes them think, all the thought balloons have question marks in them.
Don’t make me think
Being an expert in online usability Krug gathered his experience in a wonderful book. This summary of key thoughts will help you to remember most important principles and laws.