It is a black plastic container for discarded trash and refuse. When thrown or swung at a zombie, the plastic tears and scatters trash over the nearby area as well as dropping a useful item, like the garbage cans and cardboard boxes. Also like garbage cans, the food found inside garbage bags has a chance to be spoiled, which will make Chuck sick.
All of the items which can appear when a weapon is broken are found in the spreadsheet data/datafile.big/breakable.csv.
The item which appears when the garbage bag is broken depends on what number the item is assigned when placed in the area files.[1][2]