Continuous integration (CI) is a practice that developers adopt to maintain their codebase in one shared repository, and they continue to develop and integrate their code in one development branch. The developers work off the development branch, and continuously commit their changes into the development branch, and so prevent the code from going stale over the period. This practice also means developers don't have to worry about code conflicts.
In earlier days, developers used to work on a feature in isolation and used to integrate their features at the end of the development cycle. This used to pose a huge risk of running into code conflicts, or features not working due to interfaces changes, and so on. CI of the code minimizes these risks, and hence the code quality goes up.
Also, with CI in place, the manual process of integration testing is reduced, as all the integration testing is automated at this point.
The developers build new features and commit them...