Primary sources are original, first-hand documents such as creative works, research studies, diaries and letters, or interviews you conduct.
Secondary sources are comments about primary sources such as analyses of creative work or original research, or historical interpretations of diaries and letters.
You can use a combination of primary and secondary sources to answer your research question, depending on the question and the type of sources it requires.
If you're writing a paper on the reasons for a certain personality disorder, you may read an account written by a person with that personality disorder, a case study by a psychiatrist, and a textbook that summarizes a number of case studies. The first-hand account and the psychiatrist's case study are primary sources, written by people who have directly experienced or observed the situation themselves. The textbook is a secondary source, one step removed from the original experience or observation.
For example, if you asked what the sea symbolized in Hemingway's story "The Old Man and the Sea," you'd need to consult the story as a primary source and critics' interpretations of the story as a secondary source.