A reference is used to indicate the source of the information you present. There are many ways of making references and if you are in doubt about how you should do it, you should ask your teacher.

A very common way of doing it is to write (Hanson 2009) after a passage based on a text published in 2009 by a person whose last name is Hanson and then at the end of the text have a more detailed reference list. References are used together with quotation marks