Odoo Community Edition is licensed under LGPL-3. This means you can freely use, modify, and distribute the software. No license fee, no limits on number of users, no lock-in mechanisms. The code is open for everyone to review.
Why does it matter?
You own your data and your installation. If Odoo SA were to disappear tomorrow, you could continue using Community Edition. You can hire any developer for customizations, not just a certified partner. You have no vendor dependency.
Compare with SaaS alternatives: if the vendor shuts down the service, or doubles the price, you have limited options. With open source, you're free.
Community contributions
OCA maintains hundreds of modules. Individual developers contribute bug fixes, new features, and translations. Odoo 18 CE contains contributions from thousands of developers worldwide. The collective knowledge is greater than what any single company could build.
The CE vs Enterprise tension
Odoo SA develops Enterprise features not available in CE. Some in the community see it as CE being intentionally limited to drive Enterprise sales. Others see it as a reasonable business model: free version for basic needs, paid version for advanced requirements. The truth is probably somewhere in between.
What it means in practice
CE gives you a complete business system at no cost. If you need more, you can pay for Enterprise, hire a developer, or use OCA modules. The choice is yours. And that's the whole point of open source.