They're just a little decorative gizmo to put on your MagicBand. You'll still need your DVC Member card for any applicable discounts on dining/shopping.
They are called sliders. You will see other styles for sale in the parks/resorts. I like to position mine over the MB connection...just as a bit of extra security.
If you are AP or DVC and didn't not receive the slider next time you are staying on property just ask when you check in. Each DVC resort should have some extras of the DVC ones lying around and I know a few places have some AP ones.
I do wish they would have made one for every year. The first year they gave out 2013 sliders to anyone staying on property. I had hoped to start collecting multiple year ones but once 2014 started no more free year sliders