Two site-wide initialization files, `site-load.el' and
`site-init.el', are loaded into Emacs and then `dumped' if a `dumped'
version of Emacs is created, as is most common.  (Dumped copies of
Emacs load more quickly.  However, once a file is loaded and dumped, a
change to it does not lead to a change in Emacs unless you load it
yourself or re-dump Emacs.  *Note Building Emacs: (elisp)Building
Emacs, and the `INSTALL' file.)

what is "dumped copies of emacs" ?
thanks