A s'more is a traditional campfire treat popular in the United States and Canada, consisting of a roasted marshmallow and a slab of chocolate sandwiched between two pieces of graham cracker.
S'mores are associated with recreational camping. Part of the enjoyment of this simple dessert is the way in which it is made on such camping trips. A marshmallow is skewered on the end of a long stick and held just above the campfire until (according to personal preference) its outer surface starts to brown, char, or even catch fire. Once heated, the inside of the marshmallow becomes soft. The marshmallow is quickly pinched off its stick with the waiting graham crackers, one of which has a piece of chocolate on it (typically a section of a Hershey's Milk Chocolate bar). Ideally, the heat from the roasted marshmallow partially melts the chocolate into a gooey mess.
S'more appears to be a contraction of the phrase, "some more". Because once you have one you always want s'more.