The main thing that is distinctive about a secondary source in historical research is that a secondary source is narrow gauge but is not firsthand information. Let us look at the differences between primary, secondary, and tertiary sources to understand what makes secondary sources distinctive.
Primary sources are sources that come directly from people who observed history being made or participated in the making of that history. For example, an inaugural speech that a president delivers is a primary source. A diary entry from a person who heard the speech is also a primary source. These are sources which come directly from people on the scene.
Secondary sources are one step removed from this. If we look again at speeches, a secondary source might be a set of commentaries and analyses of a given president’s speeches. A secondary source might be a book about how people responded to those speeches.
A tertiary source is less focused. It might discuss, for example, the entire presidency of a given president. It does not concern itself with narrow topics exclusively. Instead, it takes a variety of secondary and primary sources and synthesizes them.
Thus, a secondary source is distinctive because it is relatively narrowly focused but is not the work of someone directly involved in an event.