It’s when you feel like you aren’t good enough - you are a fraud and just faking being competent and own day everyone will find out. Usually it’s in the context of a job. For example, if you’re a business executive in a big meeting with company bigwigs giving a presentation - you feel like you actually have no idea what you’re talking about. I’m a teacher and because of all the responsibility and many expectations of being a teacher - it’s very common for teachers to feel like they aren’t really doing a good job. Like when I write report cards, I will say to myself ‘who am I to say if this kid is meeting or not meeting expectations?!’ And I have to keep reminding myself that I am a trained and experienced educator.