Wikipedia calls The 5 Whys “...an iterative question-asking technique.” The idea is that you repeatedly ask yourself, or your client/partner/product owner ‘Why?’ as a way to better understand the issue you are addressing. It’s a useful skill when interviewing clients about their feedback or goals for a design. I can't overstate the value of this tool. I use it working with clients, in interviews with people I admire, with my students to uncover the problems they're trying to solve and even in vanilla (non-design) relationships.
Think of it as a strategy for converting an opinion or vague goal into something you can use. Yes, you'll sounds like a child, or a therapist, but there's a reason they ask questions. They're looking for answers. And if you're doing your job as a designer, so are you. This tool is so simple you can use it everyday to extract specific, actionable, details from general statements. This is also useful for negotiating creative space for yourself when a client is getting too specific. NOTE: Sometimes your Why may be better suited as a What or a How. Use your interrogatives and dig for information!
Client: "Make that button blue."
1. Why? Because I like blue.
2. What about blue do you like? I don't know. I just like it.
3. What does blue represent to you? Uh, the sky. And water.
4. What do you like about the sky and water? They're relaxing.
5. Why is that important? Because I want the user to feel relaxed.
Action: Now that you know the goal is a relaxed user you can ask the client whether they're okay with alternate solutions as long as they meet that goal.