Secondary citation is needed when you want to mention information referred to in another source.

For example:
- you read Source A, published in 2008;
- you found on p.14 information about Source B;
- you want to use this information about Source B in your academic term paper.

For example:
- Source A is a book written by Lee and Fung;
- Source B is a 2006 journal article written by McKenzie;

then your in-text secondary citation can be:

According to McKenzie (2006, cited in Lee and Fung, 2008, p.14), …