This is the best question (and variants of it) that I have asked throughout my career. I was taught the use of it as part of a Solution Focussed Brief Therapy approach to moving forward in 'stuck' situations.
I ask myself this question before trying to explain an outcome I am seeking - ' Hmmm... what will it look like when I am happy that that task is finished...?'
I ask it of others when they try to tell me how great or awful some outcome may be - 'So you are saying if you spend twice as much on that gadget, it will be better for me too... what will that look like from where I'm sitting?!" (I have a weakness for tech... I don't usually take too much convincing!)
I use it when I am being asked to take on a task that is vague or unclear (to me!) - "So you would like me to do X. What will that look like to you? What will you notice that will help you know I have done what you wanted?'
I also use it when I think a situation needs a rethink, when it seems we are all just motoring ahead without really examining enough detail! 'So what are the success criteria - what will we need to see change to know we have achieved this goal?'
I use it with children (and adults) to help them think without hinderance of problems - "So if I woke up tomorrow and everything was as good as it could be, what would I see that had changed? What would I notice?" In Solution Focussed Brief therapy this is often referred to as the 'miracle question'. With children I often introduce a fairy with a magic wand that flew over in the night, mainly to help the children engage with the question, but it also helps my imagination too!
Try it. Next time you are unsure or need clarity, ask yourself or others - 'So what will that look like?' The answer will likely show you the way forward.