A secondary source discusses, analyzes, and interprets a topic. Typically, in history, a secondary source will cite both primary sources and other secondary sources used to develop the author's argument. A historian's work, either a book or journal article, typically is a secondary source . Your senior thesis will be a secondary work.
You will want to use secondary sources for a couple of key reasons: 1) to gain a clear understanding of the events, time, people, places for a topic, and 2) to use the work cited to identify key primary sources.
Secondary sources are contextual. So, for instance, a newspaper article may be primary source for one topic (an first hand account from the time being studied) OR if could be a secondary source (discussion of an event that occurred in the past).
The databases listed here are places to start when searching for secondary sources.