The back shield is a item returning since Smash 4. It’s based on the item of the same name from Kid Icarus: Uprising, which was also created by the same guy behind Super Smash Bros.
One of the best items to have, a great tool for defense. Once you grab it, it’s yours until it times out.
And with it, nobody (unless they have a Final Smash) can hurt you from the back.
Now your front is totally open, so you have to be careful.