A "side hustle" is an additional income source that is separate from your paycheck. It's not a part-time job as much as it's something you create, and it's disproportionately satisfying. Chris strongly recommends side hustles--they help you build confidence and security, especially if you've always been an employee and have never worked for yourself. Even a small amount of extra money feels great because you made it happen.
A side hustle isn't being an Uber driver or another sharing-economy gig - that is a part-time job. No, a side hustle is an money-earning activity that fuels your passion. It can be your side hustle rather than your main career for a variety of reasons:
You need more time to prove the concept is viable: AirBnB, Twitter and countless other startups began as side hustles to the founders' main gigs. In fact, in the case of Slack, the side hustle actually was discovered within the main gig. By doing it on the side, you have the opportunity to develop the minimal viable product and find out if the idea is worth more of your time.
You need more funds: Side hustles often put passion over profit, so it is reasonable to do them as a moonlighting (or, in my case, pre sunrise) project. There may even be a point where your side hustle becomes a cash cow: My first self-published book, Damon Brown's Simple Guide to the iPad, was an experiment that ended up hitting the Amazon tech best-seller list. I ended up making more on it some months than my main work.
You need another outlet: Awful leadership, unsupportive organizations and limited growth opportunities drain employees every day, yet having a side hustle means that your main source of productivity and joy isn't just tied to one company. It doesn't mean a side hustle equals complacency at your job, but it can represent a well of creativity that encourages you to fight one more day and, ideally, get motivated to find a more supportive position.