What are [meta] issues on D.O? On drupal.org, what are exactly meta issues? Meta issues meaning issues that contain the word [meta] at the beginning of the issue queue on drupal.org.

Issues that contain other smaller issues?
'meta' loosely means 'about', like the discussions on the meta site here. They are about the site, not drupal. Meta Tags are about the page, placing it in context. The [meta] issues are not about functionality, but rather about policy, code-cleanup, documentation, standards etc., rather than regular (functional) issues which are about achieving a result through code.