About string header

[Xue Fuqiao]

In (info "(elisp) Garbage Collection"):

 -- Command: garbage-collect
[...]
So an overall result is:
          ((`conses' CONS-SIZE USED-CONSES FREE-CONSES)
           (`symbols' SYMBOL-SIZE USED-SYMBOLS FREE-SYMBOLS)
           (`miscs' MISC-SIZE USED-MISCS FREE-MISCS)
           (`strings' STRING-SIZE USED-STRINGS FREE-STRINGS)
           (`string-bytes' BYTE-SIZE USED-BYTES)
           (`vectors' VECTOR-SIZE USED-VECTORS)
           (`vector-slots' SLOT-SIZE USED-SLOTS FREE-SLOTS)
           (`floats' FLOAT-SIZE USED-FLOATS FREE-FLOATS)
           (`intervals' INTERVAL-SIZE USED-INTERVALS FREE-INTERVALS)
           (`buffers' BUFFER-SIZE USED-BUFFERS)
           (`heap' UNIT-SIZE TOTAL-SIZE FREE-SIZE))
[...]
    STRING-SIZE
          Internal size of a string header, i.e., `sizeof (struct
          Lisp_String)'.

What does the "string header" mean here?  I had searched the manuals and source 
files in src/ but I didn't find anything useful.

-- 
Best regards, Xue Fuqiao.
http://www.emacswiki.org/emacs/XueFuqiao