What Are Your Thoughts on Concept Testing?

I'm going to be doing more, more group sessions. I have some cool themes like generative research, mastermind and insights masterminds so that they're a little bit more themed. But anyways, I'm just babbling on, which I could just, could honestly do except I have a 30 minute max within this app, which is really interesting. So I have a wonderful question that came in that is, what are your thoughts on concept testing? What is it actually? When do you use it? How do you used it? All of those kinds of questions.