Basically it means that Safari will outright crash and then quit fewer times; instead it will recover somewhat gracefully by reloading *all* the web pages instead of crashing completely because of one bad page or plugin.....
...or some variant thereof...
from:
Jul 31, 2011 6:13 AM
To understand why this comes up you need to understand how the Safari processes have been redesigned for version 5.1
Historically plugins have been a leading cause of many a Safari crash. So in Safari 4 Apple seperated Safari into 2 processes - one of Safari itself and it's renderer and the other for it's plugins to run in. This meant that if the plugin crashed it Safari would in theory continue to function.
This of course ignored the other ways in which a web page can crash a browser so in Safari 5.1 Apple separated things even further. Now Safari is separated into 3 processes - The Safari app comprimising of the windows , menus , tabs , bookmarks and URLs of what's loaded in each tab/window , our old friend from above WebkitPluginAgent and a new process just for the web page renderer. This new process is called "Safari Web Content"
What your seeing here is when the Safari Web Content process crashes on the content of a page for any reason.
Safari can detect when this happens and automatically restarts the process. When you click back into a window or tab where you had web page stuff loaded before it gets reloaded.