The interview process is a series of steps that help companies assess candidates for a job. It can include multiple interviews, background checks, and other steps.
"What was the one thing they asked you?" is a question that typically means someone is asking you to recall the most important or unique question that was asked to you in a conversation, usually in a job interview or a situation where you were being questioned by someone in authority. It's essentially asking you to highlight the single most memorable question from the interaction; so, if you were asked several questions, you would pick the one that stood out the most to you.
Example answer: "The one thing they really pressed me on during the interview was about a time I had to overcome a major obstacle in a previous project and how I managed to turn it around."