What are the diagonal lines labeled "DEPLOY"?

Looking at [branches picture][1] from [GitLab Flow page][2], I guess the circles are commits and the vertical lines point to the parent commit.
What are the diagonal lines labeled “DEPLOY”?

I am reading Pro Git and it doesn’t use diagrams like that.

Thank you.
[1]: https://about.gitlab.com/images/git_flow/environment_branches.png
[2]: https://about.gitlab.com/2014/09/29/gitlab-flow/