Secondary sources are sources you have read about through the works of others. For example an article you are reading by Nicholson discusses a diary written by Allport (but you did not read Allport’s diary first-hand, you read Nicholson’s account of Allport’s diary).
Allport’s diary (as cited in Nicholson, 2003)…
The text as cited in alerts the educated reader that you have only read Nicholson’s work. Nicholson goes in the reference list not Allport.
(American Psychological Association, 2010, p.178)