Hreflang annotations tell search engines that several URLs are the same content in different languages or regions, and which one suits which audience.
What a tag contains
A language code, optionally a region code, and the URL of that version. en-gb is English for the United Kingdom; en alone is English generally.
Generating rather than typing
A site in six languages needs each page to reference all six versions, including itself. That is thirty-six annotations for six pages, and hand-writing them is how errors enter.
The rule that catches everyone
Annotations must be reciprocal. If page A points to page B, page B must point back to A, or the whole set is ignored.
Try it: Hreflang Tag Generator on SeoWolf's Notepad.