Don't make me think.
It means that as far as is humanly possible, when I look at a Web page it should be self-evident. Obvious. Self-explanatory.
The point is that every question mark adds to our cognitive workload, distracting our attention from the task at hand. The distractions may be slight but they add up, especially if it's something we do all the time like deciding what to click on.
The list dozen of things that users shouldn't spend their time thinking about, like:
- Where am I?
- Where should I begin?
- Where did they put _ ?
- What are the most important things on this page?
- Why did they call it that?
- Is that an ad or part of the site?
Based on the Steve Krug's book "Don't make me think"