weighted sack used to set climbing or rigging lines in trees. Usually a shot-filled. teardrop shaped bag.
throwbag (Wikipedia)
A throw bag or throw line is a rescue device with a length of rope stuffed loosely into a bag so it can pay out through the top when the bag is thrown to a swimmer.
A throw bag is standard rescue equipment for kayaking and other outdoor river recreational activities.
A throw bag is used to rescue someone who is swimming down a river after capsizing their kayak or canoe, but can also be used for gear retrieval and for climbing during portages.