Continuous integration, often shortened to CI, means regularly combining code changes and automatically checking them. It is like a shared workshop where each new piece is tested to make sure it still fits.
When someone shares a change, a CI service might run tests and check that the project can be built. The team can see a result before deciding whether to include that change in the main version.
CI helps find problems earlier, but a green check only means the configured checks passed. It does not prove that every possible bug is gone.