What is GitLab?
A web-based Git repository that provides free open and private repositories, issue-following capabilities, and wikis. It is like GitHub.
.gitlab-ci.yml (file)
This file defines scripts that should be run during the CI/CD pipeline. When you add the file to your repository, GitLab detects it and an application called GitLab Runner runs the scripts defined in the jobs within it.
Projects
Individual repositories.
Groups
They are directories/folders in GitLab. You create (and name) a root directory and then you can create sub groups. Then you can organize your Projects into groups.