In your research you will often encounter resources where an author refers to another researcher's work. This source within a source is known as a 'secondary source'.
To cite a source from a secondary source (“quoted in . . .”) is generally to be discouraged, since authors are expected to have examined the works they cite. However, it is not always possible to obtain the original publication - it may be published in another language, or in a book or journal which is out of print. If an original source is unavailable, Chicago requires that both the original and the secondary sources must be cited in the text and only the secondary source is listed in the reference list.
For example, Louis Zukofsky’s article from 1931 is being quoted in Bonnie Costello’s 1981 book on page 78. If Louis Zukofsky’s work was also relevant to your own assignment you might decide that you also need to refer to Zukofsky’s article in your writing. Where possible, you should try to obtain the original published work by Louis Zukofsky - in this case a 1931 journal article - and read through the article to form your own opinions on his research. However if it's impossible to access the original article, you must cite both the 1931 article and the 1981 book.
So the example above will be cited as below.
In Louis Zukofsky’s “Sincerity and Objectification,” from the February 1931 issue of Poetry magazine (quoted in Costello 1981) ...
Costello, Bonnie. 1981. Marianne Moore: Imaginary Possessions. Cambridge, MA: Harvard University Press.