Missing explanation on how to use ref_name on https://docs.gitlab.com/ce/api/commits.html

Summary

See https://docs.gitlab.com/ce/api/commits.html. The ref_name is shown, but not explained. What is it for? Where should it be placed in the URL?

Expected behavior

1. Indication where to part ref_name in the URL.
2. Preferrably a sample.
3. Is it to show the commits from a specific, like GitHub "sha"?