As we learned at the beginning of Massekhet Bava Kamma (2a), there are four avot nezikin, each with its own set of rules. While Rav and Shmuel disagree about how to define the parameters of some of these cases, both agree that one type of damage is shen – when an animal eats someone else’s property in a normal way (according to Rav, shen is part of shor; according to Shmuel it is the case of mav’eh). The Mishna on today’s daf offers a definition of shen.
According to our Mishna, as long as an animal is in the public domain, its owner will not be responsible for damage done when it eats normally (e.g. if it eats fruits or vegetables that were placed in front of a store for sale). If the animal enters a private area, however, the owner of the animal will be held fully responsible for damage done by his animal when it eats in a normal manner. If it acted abnormally (e.g. it ate clothing), the owner will pay only half damages, since it was an unexpected action. The Mishna continues, however, and explains that even in the public domain the owner will be obligated to pay for fruits or vegetables that his animal ate, if the animal benefits from the food.
From the Mishna itself, as well as from the Teshuvot ha-Rashba, it is clear that the obligation to pay in this last case is not restitution of damages, since the owner has no responsibility for shen in the public domain, rather the owner pays for the food because he has benefited from another person’s property (he will not need to feed his animal), and a person is not allowed to derive benefit at someone else’s expense without paying him.