A big part of the new Facebook Timeline is how easy it is for someone to explore all the way back through your archives of updates and pictures. Thinking back through the history of posts, you might not be sure on whether these posts are all public, or shared outside your main list of friends.
Facebook now has a privacy option called "Limit the audience for past posts". This gives you a single button to reset all of your past posts to a standard privacy setting of "Friends". This means that only those in your friends list will see all of your past posts.
It would be good if there was a better set of options here for the reset — like making your history private, or making your history only visible to a sub-list of your friends — but if you're not sure, and don't want to trawl through your whole archive double checking each and every update, then it's a handy way to reset to a standard level of sharing.