Primary evidence is evidence that actually comes from the time being studied in the question. The memories of survivors would be primary evidence. Film of the ship leaving harbor would be primary evidence. Secondary evidence is the opposite of primary. This is evidence that does not come from the time being studied.
Historians frequently produce secondary evidence.
Though she used a vast number of primary sources in her research, the actual finished book would have been secondary evidence. Each has its own value. They might exaggerate certain points, their factual recall might simply be poor etc. If someone cannot fully remember something that affected them, they might be tempted to add things in to their story so that the finished product is complete. But the finished product might also be inaccurate.