A hanging indent is a format in which all of lines except the first one are indented from the left hand margin. In APA style, citations are double spaced and formatted with hanging indents.
Example:
Simons, M., Bernaards, C., & Slinger, J. (2012). Active gaming in Dutch adolescents: A descriptive study. The International Journal of Behavioral Nutrition and Physical Activity, 9(1), 118-128. doi:10.1186/1479-5868-9-118
There are a number of ways to format a reference with a hanging indent.
In Microsoft Word, the easiest one is to select the reference and press CTRL + T.
Tip: To double space a reference (or any other text) select it and press CTRL + 2.