YAML frontmatter is a block of YAML placed between triple-dashed lines at the very top of a text file, setting variables or properties for that file such as its layout, date or tags.
Jekyll, a static site generator, states in its documentation: "The front matter must be the first thing in the file and must take the form of valid YAML set between triple-dashed lines." Jekyll processes any file containing such a block as a special file. Between the dashed lines an author can set predefined variables, including layout, permalink and published, plus date, category and tags for posts, or create custom ones, which then become available through Liquid tags in the file and in the layouts or includes it relies on.
YAML ("YAML Ain't Markup Language") is described on its official site as "a human-friendly data serialization language for all programming languages." The latest specification listed there is revision 1.2.2, dated October 1, 2021.
In 📝Obsidian, note properties "are stored in YAML format at the top of the file," according to its help pages, written between --- lines; the same pages say properties "allow you to organize information about a note." The core plugin 📝Obsidian Bases builds database-like views from a vault's 📝Markdown files and their properties, which can be viewed, edited, sorted and filtered.