Continuous Integration (short CI) is the practice of merging all developer working copies to a shared mainline several times a day.
So for every proposed change to the source code the following is done automatically:
After the system is done with all these tasks the developer can view build reports highlighting occurred errors and problems. We are using GitLab CI as our CI system.
All of this automatically kicks in when you open a Merge Request on GitLab. You will notice a pipeline block at the merge request page:
Click on the pipeline link or the individual pipeline stage icons (circles) to find out the reason for a failed check. If you add more commits to this merge request all checks are run again.
See the GitLab CI page for more info.