Summary: The more choices you are given, the harder the decision will be for you.
Elements of User Experience series: "Hicks Law"
"Hicks Law: the time required to make a decision increases, with the additional number of alternatives you are presented with"
The principle is: a) you are presented with a task / goal / issue, b) you analyse the situation, judge your options to achieve the given task c) you make a decision / choose an option d) you apply / execute your decision. In this context Hick's law predicts (algorithmically) that the more alternatives you provide users with the harder you are making things for them. You should aim at presenting users with only the options they require to achieve the task.
Context in UX
In a moderately complex to simple situation, like with a website design, Hick's law can be an interesting principle to check. Do you really need all that content / all those options / all those images to help the user achieve their goal? As explained in further detail by Smashingmagazine, it usually makes sense to "take a step back" and think of your project in general rather than applying this principle religiously to each sub part and element of the overall structure one by one. Consider the user journey, the objectives and via testing if necessary clarify that certain elements are not superfluous.