Why there is a (') prefix on "default" and "current"?

[Dancefire]

Hi,

When I translate for string
-----
A list of directories (strings) indicating where to look for html and
parsed-html files. Each element must be a string representing a
directory or a symbol where 'default expands to the default path, and
'current expands to the current value of the path.
-----
in src/scm/command-line.scm

I first thought, it's single quote for string 'default expands to the
default path, and '. But then, I realized it's not. It looks like a
prefix of word "default" and "current". So, is there any special
meaning of it? or a typo?

Regards,
-- 
Tao Wang