That's cabin insulation - probably fiberglass.
It's not always in green bags (blankets), and not always fiberglass, but some kind of insulation is necessary to help control temperature and reduce cabin noise (air moving over the fuselage is REALLY loud).
Here are some other photos of different types of insulation:
Basic fiberglass with a plastic backing, cut to fit.
Fiberglass with either plastic or foil backings is commonly used because it is both light and inherently flame-retardant (as long as the backing is fireproof it will not burn).
Soundex - a sandiwch of acoustic foam, thermal insulating foam and aluminum foil - is a popular choice on propeller aircraft: Slightly heaver than fiberglass, but better at dampening sound.
Foam padding (similar to the foam used under carpets) does a decent job at both sound and thermal insulation. In this case it's green & has no covering, but colors and coverings vary.
Prefabricated insulation blankets on a Boeing 777.
(I believe these are fiberglass sealed in polyethylene, they're manufactured to fit each bay without cutting which avoids loose insulation material floating around).