What is it when is a question frame for a situation you can describe but cannot name. It asks someone to supply the missing word or explanation.
My knee hurts only when it rains. What is it when that happens? The speaker pictures the situation clearly and just wants the label, and the listener would supply one, like weather-related knee pain or weather sensitivity.
A customer points at a phone screen and asks, What is it when my phone shows a tiny battery with a red line? The clerk answers, A low-battery warning. The customer described the situation; the clerk supplied the label.
Someone asks, What is it when you can't stop checking your messages, even at dinner? A friend answers, That's a habit, or maybe phone addiction. Again, the description was complete and only the word was missing.
What is it when bread turns brown in the toaster? asks for the missing term, the word browning.
So the frame fits whenever you can picture the situation but not the word for it: if you only want the label, a plain naming question works.