Self-employed people are entrepreneurs of one kind or another who are their own bosses and work for different clients on a project basis, or are working on creating a product or service that brings them revenue.
Being self-employed is an arrangement that may work in a number of different sectors. Contractors and tradesmen, web and app designers, software developers, social media managers, YouTubers, business owners, illustrators, writers... The list is not exhaustive, by far. Their common denominator is that they arrange every aspect of their operation themselves. They find clients, carry out the job, maintain their own equipment, and submit their tax returns. Okay, sometimes they hire help for some of the tasks.
You will manage your working hours yourself, which will probably mean they end up being all over the place. But this is a small price to pay for being able to do something you really love, to see the results of the energy you put in and watch yourself become better, stronger and more independent and empowered every day.