In my humble opinion, I'd stay away from shield cents if looking for doubled dies. Or really anything after 1995.
Here's a link on doubled dies! good luck! http://www.lincolncentresource.com/doubledies/doubledies.html
That happened when it was struck by the dies. It's strike doubling. These are violent, rapid strikes, and the planchets that aren't braced enough for the impact twist and hop and slide just enough to leave those impressions behind on the coin.
MY humble opinion on "what I am looking at on my coins?" is having fun with the hobby. Some folks ONLY collect Errors. Some only collect wheat Pennies, while others collect a little of everything. I say Have fun and do what fits YOUR fancy with coins.