Restart screen features a progress bar: what is it? Sometimes (and in particular the last two times I restarted), a strange bar looking like a load bar or a copy bar appears on the restart screen, as in the image below.

I am running Mac OS X 10.9.5. What is it? Is it just the installation progress of updates (that install without me saying anything but that is another story) or is it something else?
Note
It has oftimes happened to me that I saw this:

Is it correct to call that &quot;freezing&quot;? If not, what is the correct name? And is it any different from hibernation aka deep sleep?
This is the Mac restoring from safe sleep (aka hibernation). The contents of RAM was previously copied to disk and is now being restored. For HDDs, this process can take a while.


  When your Mac wakes from sleep, you might see a progress bar on its display. This is normal behavior that means your Mac is waking from Safe Sleep.


If you see a progress bar when your Mac wakes from sleep - Apple Support