The Treaty of Versailles in the mind of the Germans stabbed them in the back, especially the guilt clause that made them take blame for the First World War. Hitler capaitalized on these sentiments and used them to rally the German people.
Primary sources are the first hand evidence left behind by participants or observers at the time of events. Primary sources originate in the time period that historians are studying. They vary a great deal.
They are called secondary sources because they are created after primary sources and they often use or talk about primary sources. Secondary sources can give additional opinions (sometimes called bias) on a past event or on a primary source.