Continuous integration allows ensuring the quality of small code parts
CI is the process of running unit tests and automated interface tests on every code change (typically each commit). This is great because it provides near-constant feedback to your developers, so that if they break something during development they know about it immediately and can fix it before it gets further down the line to a review phase.
Developers upload the smallest changes in code to the central repository, and whenever the code is updated, we have a series of tests performed to ensure the top-quality of each code segment.