Hi, it is understandable, but it's not as precise & clear as it could be. There are many different ways it could be worded. Consider the following options. If speaking to an educated native speaker: What constitutes a successful person, in your eyes? If speaking to an unintelligent native speaker, or a non native with weak English: What IS a successful person? Somewhere in the middle: What's your definition of a successful person? Or: What does it mean, to be a successful person?

It is fine, but it's essential that you place "a successful person" in quotes. If you do not do this, the sentence will not be understood. If you are speaking rather than writing, you must articulate the words "a successful person" in a way that demonstrates it to be a quoted term.

The sentence "What is the concept of a successful person?" is perfectly correct and sounds natural. It's a common way to ask about people's definitions or perceptions of success. So, you don't need to worry about it being wrong.