"The devil's advocate" is someone who pretends, in an argument or discussion, to be against an idea or plan that a lot of people support, in order to make people discuss and consider the idea in more detail.
At first, you might be wondering - why would I need a person who defends a position that's opposite to the decision of consensus? Doesn't it put everyone else in "defensive" positions? Doesn't it lead to conflicts?
And that's usually a good thing (when it comes to collective decision-making).
The devil's advocate will ask questions that nobody else dares to ask, so helps to break conformism and "anchoring" of decisions (everyone agreeing to what was first said because it's easier and less scary).
The devil's advocate will help everybody brush up on their negotiation skills, finding the correct arguments to defend a position. If such arguments are too weak, is it a good decision in the first place?
New ideas will spark or variations of the original idea will emerge.
Next time a new idea is getting bounced around in a discussion, try being the devil's advocate yourself.