A secondary reference is when you refer to someone cited within another source, i.e. you have not read the original work. Ideally, you should always try and read the original source so that you can review the work first-hand. If you are not able to locate the original source, it is acceptable to reference it as a secondary reference, following the format below.
Example: in text citation: (Bancroft and Silverman, 2002, cited in Harne and Radford, 2008, p.63)