'Meta' is the governing meta site for all Stack Exchange sites together. Use this tag to ask questions about this very site, a.k.a. MSE. In some sense, the [meta] tag is Meta-Meta Stack Exchange.
Meta Stack Exchange a.k.a. MSE or 'mother Meta' is the capital of all of Stack Exchange sites.
You can think of the questions and answers under the meta tag to be Meta-Meta Stack Exchange.