A print source is one that has been published in a physical print format such as newspapers, magazines, journals, books, or government documents. Note that print sources are not always used in their original physical print format. Sometimes print sources are available online such as a newspaper or journal article found in an online academic database.
What distinguishes a print source from a strictly online source is that it is available in a physical print format, however you may be able to access and use either the physical print or online format of the source.