but, WRT abbreviations, it's always explained that "*"
(again short for child::*, right?) will select elements.
unless it's used in conjunction with the attribute axis,
at which point either attribute::* or @* will select just
attribute nodes.  same for the namespace axis.

so what exactly does "*" mean?

It doesn't mean anything specific beyond the semantics assigned by
the spec.
If you really want to have an analogy, take it as the famous wildcard
matching "any string", or matching everything named on the respective
axis. Elements and attributes have names and can be matched; text,
PI and comment nodes don't have names. I'm not sure about namespace
nodes though.