Someone asked about the phrase "diamond open access" on the journal's home page.

Gold open access is where authors pay to publish in a journal and their article is then free for others to read. The journal is usually peer-reviewed.

Green open access is where authors put their work in a repository that's free for others to read - like the arXiv, or PubMed. This is often done in addition to publishing the paper in a journal, since these repositories are typically not peer-reviewed.

Diamond open access is where authors publish in a peer-reviewed journal that's free for others to read, and the authors don't have to pay to do this.