General Average is a legal principle of Maritime Law under which, all parties who are involved in that voyage, shall be asked to proportionally share the losses resulting from such sacrifice..
All parties including merchants whose cargoes landed safely would also be called upon to contribute a portion (based upon a share or percentage) to the affected parties which could be the shipowner, merchant or merchants whose goods were jettisoned or damaged in the process of preserving the safety of the ship and/or crew..
Say for example the ship is a container ship and there are 100 containers on board with 100 customers.. One of the containers caught fire on board which spread to 9 other containers and all 10 containers had to be thrown overboard in order to save the balance 90 containers, the ship and the crew..
Since the ship, the cargo and the crew were saved due to this action, the whole burden of the loss will be shared among the 100 customers and not just the 10 customers whose cargo was thrown overboard..
General Average was formally codified into the York-Antwerp Rules, in 1890, but has been updated several times, with the most recent update being 2004..