Full explanation
A side project is something you build during evenings and weekends, while keeping your main income. It is the safest way to test entrepreneurship without giving up your salary.
Many successful startups began as side projects: Twitter, GitHub, Gumroad. Working in small windows of time forces you to focus on what really matters.
The hard part is energy management. You only have a few hours per week, so every shortcut and every existing tool counts.
Concrete example
You are a marketing manager during the day. On evenings, you build a small tool to help freelancers send invoices. After 6 months, it brings you 2 000€ per month.