In academia, a primary source contains original research whereas a secondary source provides information about or refers to a primary source.
Sometimes you may read about research by another author (the primary source) in a secondary source and you want to include this primary research in your work. It is best academic practice to find and read the primary source and then cite this directly in your work. However, this is not always possible (the book may be out of print or the article unobtainable). If you are unable to get hold of the primary research, then you can use secondary referencing in your work - although it is is recommended that you do so sparingly.
If you do need to provide a secondary reference you need to:
If the year of publication of the primary source is known, also include it in the text citation.
So, if you wanted to cite research by Jiang (2010) which you had read about in Grieg et al. (2013) but you were unable to read Jiang's work, in the in-text citation you would cite Jiang's work as the original source followed by Grieg et al.'s work as the secondary source. You would only include Grieg et al.'s work in the reference list (as this is the work you have read).
You could cite this in-text as:
(Jiang, 2010, as cited in Grieg et al., 2013)
The reference list entry would be:
Greig, A., Taylor, J., & MacKay, T. (2013). Doing research with children: A practical guide.
For more information on secondary referencing see the APA's official guidance at: https://apastyle.apa.org/style-grammar-guidelines/citations/secondary-sources