The idea that just believing in yourself, going out there, and faking it will lead to real confidence and long-term success. 
Confidence is the realistic expectation that you have of being successful at something given your competence at this thing and the risk involved in doing it. Humans developed confidence as a way to track our competencies so that we make the right decisions about various risks. This is not feeling good about yourself. This is not believing in yourself no matter what. That is self-esteem. 
You build competence through preparation, practice, and demonstrated performance. You can't fake competence, you actually have to work to get good at a skill. 
If you aren't good at skiing, you can't fake going down the Black Diamond run. It's the same with talking to women. If you have low social competence (trouble talking to people), you can't fake talking to women. In fact, faking confidence here will make you look even worse because people can tell that nothing is behind it. 
Faking it is good advice to follow when you are already competent and you have the skills, but subjectively you don't believe that you are any good. Your confidence is much lower than your competence. Here "fake it 'til you make it" can help you become confident when your skills are already there. 
A better strategy is to get objective feedback. To figure out if you should focus on either building skills to get better or faking it and acting as if, ask people around you, a good friend, who can be honest about whether or not you already have these skills.