About "set bits"

[Xue Fuqiao]

In (info "(elisp) Bindat Spec"):

  `bits LEN'
     List of set bits in LEN bytes.  The bytes are taken in big endian
     order and the bits are numbered starting with `8 * LEN - 1' and
     ending with zero.  For example: `bits 2' unpacks `#x28' `#x1c' to
     `(2 3 4 11 13)' and `#x1c' `#x28' to `(3 5 10 11 12)'.

I don't know what "set bits" means here.  I have searched the archives,
and found two related threads:
http://lists.gnu.org/archive/html/help-gnu-emacs/2005-06/msg00087.html
http://lists.gnu.org/archive/html/help-gnu-emacs/2013-01/msg00103.html

But they didn't help.  I have also searched the web and seen the
commentary header of bindat.el, but nothing helped to me.  Can I get a
pointer to information on this concept (or on byte packing)?

-- 
Xue Fuqiao

English is not my native language; please excuse typing errors.