Developed by Google, it is the engine running behind Google Chrome browser (hence the naming similarity) made available open source. The Google Chrome browser, while being built on top of Chromium, it is proprietary software.
So why make it available open source?
One of the main reasons is contribution from a great community of developers. There are amazing ideas coming from someone outside of an organisation that can be a big benefit for the company using that software.
Why would anyone contribute?
The reasons are simple, people contribute to develop the project further and improve. In parallel you could say that you improve google chrome indirectly but it’s not only about google chrome.
Being open source, while you read and abide to the official license, you can also create your own browser on top of it. As I’ll list further below some products derived from the chromium.
It’s not only about browsers. Some amazing tools have come out of it that can use the latest browser software in order to solve some really important problems such as e2e testing (end to end testing) through a headless browser module using Chrome Dev Tools Protocol.
This is a shorter article focusing straight on what is Chromium, please let me know if I forgot to include anything.
https://www.chromium.org/chromium-projects/ – extract docs