Every page in your space is available via the page tag. You can attach additional tags to a page, by either specifying them in the tags attribute Frontmatter, or by putting additional Tag|tags in a standalone paragraph with no other (textual) content in them.

In addition to ref and tags, the page tag defines a bunch of additional attributes as can be seen in this example query:

${queryfrom index.tag "page" where name == _CTX.currentPage.name}