What is it?
You are asking questions and try to learn from the user about the subject which you work on.
How can I use it?
Set a clear goal which you want to achieve at the end of the interview. Make the interviewee feel comfortable. This is not a conversation or dialog, let him/her to speak mostly. Prepare questions before.
When can I use it?
You can use it at the early stages also later on. In early stages you can get data for Persona, Customer Journey Map etc. Also you can use it after testing process.