Primary source reprints are collections of carefully selected historical documents. Each document in such a reprint is chosen by scholars for its value in researching a specific field of inquiry, especially fields of inquiry not well supported by other, readily-available primary source collections. Primary source reprints are often issued to fill a perceived void in the available body of source documents for research in an emerging field of inquiry (for example: history of suicide; history of sexuality).
You can expect the documents published in these collections to be documents that scholars consider most important or especially rich in potential for supporting historical research. The collections will often include valuable editorial apparatus, such as introductory essays that situate and contextualize the documents, as well as appendixes, indexes, and other features that will make the primary sources easier to use.
Many research guides recommend beginning a research project with a literature review of secondary sources, and then searching for primary sources after. An alternative approach, however, is to begin your research by identifying a body of primary sources that you intend to study. (Be sure to consult with your professor first, to ensure that this approach will be appropriate for the assignment you are working on.) Primary source reprints are excellent tools for beginning a research project in this fashion: browse the available titles, find one that interests you, and you will instantly have a collection of highly relevant primary sources.