Primary sources are the historical documents used by historians as evidence. Examples of primary sources include diaries, personal journals, government records, court records, property records, newspaper articles, military reports, military rosters, and many other things.
In contrast, a secondary source is the typical history book which may discuss a person, event or other historical topic. A good secondary source uses primary sources as evidence.
The key to determining whether an item may be considered to be a primary source is to ask how soon after the event was the information recorded. This can be a problem with an autobiography, memoir, reminiscence, etc. if the author is working several years with only the memory of what happened. Your history professor will disallow most or all of these as primary sources. However, the rule of thumb should always be: if you're not sure whether something is an acceptable primary source, ask your professor.