what does it mean if the empty string can be repeated n times?

It means "We kept the heading short, feel free to split hairs." The single line below that heading reads: An empty lower bound is now accepted for regular expression quantifiers, like {,3}.

But I'm rather expecting Perl not reading the curlies as metas.

Correct. A open curly was interpreted literally in regular expressions in a lot of places in old Perl versions. Since Perl 5.26 some of these uses were forbidden, and Perl 5.28 made some of these uses non-fatal again. Relevant reading:

• perldelta 5.26: Unescaped literal "{" characters in regular expression patterns are no longer permissible
• perldiag 5.26: Unescaped left brace in regex is illegal here in regex; marked by <-- HERE in m/%s/
• perldelta 5.28: Some uses of unescaped "{" in regexes are no longer fatal

so {,3} is the same like {0,3} or {1,3} ?

Same as {0,3}.