Footnotes are short numbered notes that are placed at the bottom of the page in an essay or article. They are used for a variety of reasons including, citing materials, providing notes on a source or topic, and to acknowledge copyright status.
Although you will find footnotes in many journal articles, they are not typically required in APA or MLA formatted essays. They are most heavily used when applying the CMOS style.
For information on footnotes in the Publication Manual of the American Psychological Association see section 2.13 "Footnotes.".
For information on using footnotes with MLA see the "Using Notes in MLA Style" article from the MLA Style Center.
For information on footnotes in The Chicago Manual of Style see Chapter 14 "Notes and Bibliography."