I'm sorry to ask such a naive question. It seems obvious that most things labeled as shields or shield kits are simply add-on type hardware, but I don't understand the use of the word and would like a little clarification.
P.S. This is my first post and there will likely be many, many to follow. I am planning on building an automatic ball launcher for my dog. Something to keep him busy during the day (and just to see if I can do it). I've a lot of experience with programming, but limited circuitry knowledge. Wish me luck.