It's sound deadener, and since it's between a brace and the roof skin probably for rattle proofing. You can reattach to a cleaned surface with a good grade of adhesive.........construction adhesive might be more economical than the spray 3M.

A light weight alternative I've used is a material available in any home supply store (Home Depot, Lowe's, etc). It's a bubble wrap like product with aluminum "foil" on both sides. Excellent sound proofing and heat reflection. Nice and light weight so it doesn't need a lot of adhesive.