A secondary source is a text you encounter second-hand whilst reading another source - i.e content first reported in another source.
For example, you may read a text by Smith and Stewart (2010) which references earlier work by Slack (2003). If you cannot locate the earlier paper but want to reference information found in it, you can create a secondary citation indicating you read Slack (2003) second-hand, i.e. via Smith and Stewart (2010):
According to Slack (2003, as cited in Smith & Stewart, 2010, p. 2)
… literature generated (Slack, 2003, as cited in Smith & Stewart, 2010, p. 2).
Cite secondary sources sparingly.
If possible, as a matter of good scholarly practice, find the primary source, read it, and cite it directly rather than citing a secondary source.
Only the source you read first-hand (Smith & Stewart, 2010) should be included in the reference list