Primary sources are first-hand accounts by participants of a particular event or materials produced at the same historical time period. Primary sources act as a form of evidence of life or thought.
Secondary sources interpret or analyze primary sources. They are a step removed from the event or original text, and act as a form of insight, context
If you are analyzing a magazine article from the 1940s to talk about what life was like during that time period, it is a primary source. However, if you are citing information from that magazine article for it's commentary on something contemporary, then it is a secondary source.