This is xss-lock, a program for using an external locker as the X
screensaver.

xss-lock can be run at startup with a favorite screensaving or
screenlocking program:

  xss-lock slock

xset values for screensaver onset are respected. More details can be
found in the man page.

xss-lock is a good choice for window manager-only graphical setups.
Desktop environments with native screensaver programs can (and probably
will) interfere with xss-lock.

Optional dependency: docutils (if not installed, the man page will be
copied in rather than built)