Running screensaver software in linux lubuntu 15.10

How exactly do it?

One way to do it is to install xscreensaver.

sudo apt-get install xscreensaver xscreensaver-gl-extra xscreensaver-data-extra

Then Preferences → Screensaver (and accept the first time message if any)
Finally, open Preferences → Default applications for LXSession → Core Applications
and fill the Screensaver text field with:

xscreensaver

thanks
& how exactly make Screenshot with the software of xscreensaver?
that i click in keyboard on key of printscreen, I don’t see like A pop-up with the Screenshot like in linux mint mate.

So you didn’t mean a screensaver as in…

A screensaver (or screen saver) is a computer program that blanks the screen or fills it with moving images or patterns when the computer is not in use. https://en.wikipedia.org/wiki/Screensaver
... but a [url=http://linuxforums.org.uk/index.php?topic=12673.0]screenshot tool[/url] to make screen captures? If so, xscreensaver is not the right tool for that job.

I could have sworn Lubuntu comes with scrot preinstalled

what happens if you hit the PrtScr key … does an image get saved in your home directory ?