(Though I am not a native speaker of English. But I guess...) This term is often used for getting the MINIMUM requirements for creating a mental picture of something.
We can use "What is" for the minimum information (as a word) and also for a very extensive explanation that contains different types of information.
Let's check whether the usage 'MINIMUM' is correct or not.
A person (especially, if he doesn't know English well) often uses/can use "What?" to mean "What is?" if he can't hear or understand the question he was asked; no matter whether it is a wh-question or a yes-or-no question. The shortest response will be "What?" to mean "What is?". Here "What?" is used just for repeating, sometimes for an explanation. So we can confirm that the usage 'MINIMUM' is correct.
Other wh-question words can be reworded using "what", I think.
When --- at what time
Where --- at what place
Who --- what person
Which --- what one or ones
Why --- for what reason
How --- in what way or manner; by what means.
(You will have to add "is" to each word given above.)
I think this is possible with other wh-question-words also.
If all these question-words (given above) can be compared to tools, "What is" acts as our hand for holding those tools. I mean, we can't use tools without our hand. In other words, other wh-question-words would be useless without the help of "what is" (as its base).
E.g.: The question, "Who is that person?" won't be meaningful if it doesn't mean "What is the name of that person?", "What is his relationship with this area/the other person?" etc.
"What is" is an alternative term that can be used to indicate any wh-question-words. So it can mean all the meanings that wh-question-words imply.
In short, it is a Panacea.