(N) sets the NULL_GLOB option for that pattern:

    % print doesnt-exist*
    zsh: no matches found: doesnt-exist*
    % print doesnt-exist*(N)
    (doesn't print anything, pattern is just ignored)

([n]) is array indexing to get entry n; -1 gets the last entry:

    % print a*
    a-1 a-2 a-3
    % print a*([-1])
    a-3
    % print a*([1])
    a-1

Globbing is automatically sorted by name.

So putting that together, things like this:

      ~/.local/share/gem/ruby/*/bin(N[-1])  # Ruby

will use the latest Ruby version in ~/.local/share/gem/ruby, and it won't error out if it doesn't exist (so I can freely copy this around even to machines without Ruby, or remove Ruby, etc).

Thanks for explaining, looks very clever. Is this syntax part of bash?

No, bash has nothing like this. It's all very specific to zsh. They're called "glob qualifiers" if you want to find out more.