Simplest way to kick off CI/CD only on master

I’m new to GitLab, and using the free (400 minutes) of CI/CD. I have a simple CI/CD that runs my test suite. But for now, I only want to do so on changes to master. What’s the easiest way to do this? Perhaps a config setting rather than custom scripting?

You can use the only keyword, ex:

      - master
  script: npm run test

more info here: GitLab CI/CD pipeline configuration reference | GitLab

Thank you! Easy as could be.