This is where an author refers to the ideas of others within a work that you are reading and you reference the source you have read, rather than the original sources mentioned by the author. To be used sparingly! It is always better to access the original source rather than relying on secondary sources.
In-text example
Name the original author with a reference to the secondary source.
Rubin’s study of romantic love (as cited in Sabini, 1992)…
Reference list
Sabini, J. (1992). Social psychology. W. W. Norton.