= the source you are reading refers to content that was first presented in another source.
For example, you have a passage in your term paper that you would like to reuse in your Bachelor thesis:
You insert the passage in your Bachelor thesis, without stating the source (which is your term paper). This is self-plagiarism, and you can be accused of cheating:
To avoid self-plagiarism, state the term paper as the source at the end of the passage:
(Nordmann, 2018, p.45)
You have not read the sources in the passage you copied for this paper (Bachelor thesis), but for your old term paper. You must change the citations in the passage to show that it's a secondary sources (see definition in the right hand menu).
In the text from your term paper you have the source Bandura, 2010:
Change the source in the passage to:
(Bandura, 2010, as cited in Nordmann, 2018, p. 45)
In the reference list use your term paper as your source – not Bandura.
Remember that secondary sources should be used with caution, for example when the original work is impossible to find or otherwise inaccessible, or in a language you do not understand. It's best not to quote yourself, but to read the primary source over again, formulate new text and only refer to the primary source.