Primary sources come from the time the historian is studying. They can be written or non-written.
Some primary sources for the First World War would be: letters and diaries written by soldiers, their uniforms and their weapons.
Buildings, archaeological remains, paintings and objects are all valuable primary sources.
Secondary sources do not come directly from the events they discuss. They are sources based on other sources. Books about the First World War by modern historians, or school textbooks about the First World War (see picture), are examples of secondary sources.
The author of this book about World War 1 has used many primary sources (both written and non-written) for his book.