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.
Example...
I don’t like this logo.
1. Why? I can’t read what it says.
2. Why? Actually it’s okay at some sizes but doesn’t read when small.
3. Why? The font closes up.
4. Why? The icon overwhelms the name.
5. Why? The icon is too big and has too much detail.
Action: Select a new font, simplify the icon and rebalance the hierarchy between the icon and the name.