Secondary sources depend upon primary sources. Secondary sources describe, discuss, interpret, comment upon, analyze, evaluate, summarize, and process primary sources. The important thing to keep in mind when trying to decide if a source is primary or secondary is whether or not the author did the thing they are reporting on. If they did, it is a primary source; if they did not, it is a secondary source.
Secondary sources represent the scholarly conversation that has taken place, or is currently taking place, on a given topic. Thus, it is imperative that researchers acquire a comprehensive knowledge of the secondary literature on their topic to be able to then engage with it and offer their own perspective through their writing. Scholars show their deep knowledge of their topic by demonstrating in their writing their awareness of secondary literature. Research that does not include substantial references to both primary and secondary sources is not likely to be authoritative or reliable. For that reason, looking at the listed references in a piece of research can help you determine its value.
Like primary sources, secondary sources can be lots of different kinds of resources depending on discipline and application. Secondary sources can be:
In the sciences, secondary sources tend to be things like literature reviews (synthesized descriptions of previous scholarship on a topic), systematic reviews (overviews of primary sources on a topic), or meta analyses (studies in which conclusions are drawn from consideration of systematic reviews).
In the humanities, secondary sources tend to be journal articles that discuss or evaluate someone else's research, monographs, or reviews.