Archetypes in Hugo are templates for different content types. Among other things, they allow you to configure frontmatter for whatever content type. They are found in either the top-level archetypes directory of your project, or if you’re using a theme, in themes/your-theme/archetypes and their the name is the content type + .md, like posts.md.
Then, when you run hugo new posts/new-post/new.md, the appropriate folder and file will be made with initial content from the matching archetype file.