That's UV protection. Is it in bad shape? It's kind of exspensive and save things from fading . To remove you'll need to try to peel it back from a corner, using a spray bottle keep spraying at it on the glass towards the fold so water can get under it and keep pulling gently. If not working try goo gone and a razor blade scraper. Not an easy process, be patient.
You will need to use a blade scraper to remove it. You will sometimes end up will a sticky residue which can be removed with Wd40.