Secondary Sources are one step removed from primary sources. They offer analyses, descriptions, or interpretations of information that was already available elsewhere (e.g. through primary sources or other secondary sources). Secondary sources can include scholarly articles, academic books, literary criticism or interpretation, biographies, histories, political analyses, law reviews or analyses, art reviews, conference papers, dissertations, etc.
Once you find a number of secondary sources, you will need to decide which of these sources you want to "invite" into your paper to have a conversation with you and the claims or interpretations you hope to make. Scholars are often in debate with each other, and that just because someone is considered an "expert" in a field, that doesn't mean that everyone agrees with their interpretations or approaches! Consider the following questions as you evaluate your sources:
Methodological or disciplinary perspective: Are these articles written by historians? Philosophers? Film scholars? Are they interdisciplinary? Each discipline can have its own "frames" of knowledge and approach to scholarship. Understanding the disciplinary frame can sometimes help you figure out what is and what is not being considered.
Who are they in conversation with? Is this article or book written in response to another work, or in response to a way of thinking within a discipline?
Who is the writer? Are they well known in their field? Why? Who are their influences? Is anyone writing against their work or approach?
Why should you choose to "invite" this particular author or work into your paper?