Citations to sources are an important part of historical scholarship. They enable authors to acknowledge their intellectual debts (and thus avoid plagiarism), as well as enable readers to check an author’s sources for themselves.
History classes typically require students to give their citations in footnotes (as opposed to endnotes or in-line with the text). They also typically require students to format their footnotes according to what is known as “Chicago style”–that is, according to the Chicago Manual of Style. This is not the world’s least interesting fashion guide, contrary to what the title may lead students to believe. Rather, Chicago style simply provides a standardized way of listing information about authors, books, page numbers, and so forth in citations, so that readers familiar with Chicago style (like your History professors) can digest the information in your footnotes more easily. The information contained in a Chicago-style citation is the same as would be contained in, say, an MLA- or APA-style citation; it is just presented differently.
Students may find this guide to Chicago style helpful: https://www.chicagomanualofstyle.org/tools_citationguide.html