Remnant of the type of keyboard that Emacs was developed on. Also the same place that the Super, Hyper, and Compose keys originate from
You can map it through your X server/Wayland compositor, but if not Emacs recognizes Alt and Esc as valid meta keys
Run
info info
Texinfo pages were originally meant to be a longer alternative to manpages that had support for featureful navigation (links, indexes, etc). They're nice and I can see a world where they did catch on, but the standard viewer is always a little bit of a shock to jump in to (being based off Emacs and all)