A secondary source is a modern author who is commenting upon and, usually, intrepreting primary data.
Primary data are the material left behind by ancient societies.
In the case of Greek myth, the primary data are very often literary sources, but they can also be visual imagery, archaeological material, etc. As you read an article, the first skill that you need to develop is the ability to sort the primary data from the secondary analysis. As a reader your next strategy is to generate a list of questions, or issues, in which the secondary source is interested. Generally, the secondary source will be attempting to answer these questions, or address these issues, via an analysis of the primary data. Data is not neutral.