If you walk away from any computer for a number of minutes some preset programing will kick in. Perhaps you have yours set to shut down to save battery life or to screen lock requiring a password to break back in.
In the old days, most of us set various assorted screen savers, especially on PCs before someone figured out that it was an excellent way to spread viruses. The built in ones usually, but not always sucked. Either there were toasters flying around, some word spinning, flipping or morphing, or the ever popular logo.
I haven’t figured out how to import screen savers to my MacAir, but it turns out not to be relevant. I just have it set to flip up a montage of entries from iPhotos. Sometimes they are whole screens, sometimes 1/2 to 1/8 screen shots in various orders. Some of them actually have turned out to be pretty interesting.
Now, I wish I could just predict what was going to come up next….