Primary historical sources are documents, publications, and artifacts contemporary with the period studied, and later accounts of participants in events. For primary information about allotment, for example, a historian would seek out the statements of people involved, such as accounts of congressional debates, reports of officials involved in making and administering the policy, statements of Indian people who were involved as subjects of the policy, observations of others with first-hand knowledge, and statistical data from the period. Secondary sources are accounts of historians and others who have used primary sources to construct accounts of events they themselves did not experience directly. Obviously, primary sources get historians closer to their subjects than secondary sources. Therefore, historians tend to give more credibility to historical writing that uses primary sources.
Secondary sources are also important, however.
A historian working on allotment would study what other historians have written about allotment as well as primary sources. Such a historian would want to know what sources others have used, and what interpretations they have made of the material. Since the point of historical writing is to add to historical knowledge in some way, our hypothetical historian would have to know what others have done in order not to waste time repeating their work. And the historian would want to be able to argue effectively against apparently mistaken interpretations. Good, current secondary sources are also useful in establishing background.