Concept introduction and purpose

GitLab Runner is an application that works with GitLab CI/CD to run jobs in a pipeline. It can be installed on your own infrastructure or used as a cloud service from GitLab. Runners are the agents that run the CI/CD jobs that come from GitLab. Executors determine the environment each job runs in. You can use GitLab Runner to run jobs over multiple machines and send the results to GitLab. To use GitLab Runner, you must choose an executor.

You can register the runner as shared group runner or specific to a project while installing it.

Register group runner

Most of the time it makes sense to create the runner on a level where many projects can use the same one. In this way, resources can be saved.
If individual runners have to be used for security or compliance reasons, this can also be done at project level.

You can find detailed instructions for different operating systems here: https://docs.gitlab.com/runner/register/