Single origin chocolate is chocolate made from just one variety of cacao bean, grown in a particular region. This has a significant impact on the flavor of the chocolate, as rather than eating a chocolate with a blend of different cocoas, you are sampling the specific flavor notes and aromas of a specific cacao bean.
Most often, single origin chocolate only refers to the cocoa used in the creation of the chocolate bar, although it is possible to find chocolate where the milk solids and sugar all come from the same region as well.