I just got braces but they put this thing on the roof of my mouth. What is it? (Photo)

I had brackets on (3 months) and extractions (2 1/2 weeks) and I finally get all off the braces appliances on but they put this thing of the roof of my mouth and i don't know what it's is . If u can tell what it is, what it does, and how I clean it that would be great!