An import package is a description of a related set of CSV records for importing. A package defines groups of records that are to be imported and the order they must be processed in. These groups are referred to as segments. A package can contain any number of segments. Each segment is considered to be an indivisible entity. The import system will import the whole of a segment or none of it. An error anywhere in the segment will cause the whole segment import to be discarded.
A package segment can either be self contained or it can refer to a separate CSV file. When self contained the actual CSV records are contained within the package file. Otherwise a reference is made to the CSV file (or part of it) that contains the records.
In summary: a package controls what records are to be imported and in what order.