The term, the Night of the Long Knives, refers to the events that occurred between June 30 and July 2, 1934 in the Bavarian region of Germany. Adolf Hitler ordered the arrest of all the members of the SA (Sturmabteilung) and the execution of its leaders. Sturmabteilung translates to English roughly as stormtroopers, but they were more commonly referred to as brown shirts.
The SA were a paramilitary group that was formed by disenfranchised combat veterans of WW I that were unhappy with how Germany was being treated as a result of the Versailles Treaty. They were an increasingly unpopular group, particularly unpopular with the German army, mostly for their use of street violence to get what they wanted. Among other things, the SA were responsible for security at Nazi Party rallies and in charge of the Hitler Youth corp. They were members of the Nazi Party but were mostly autonomous. Hitler could not control them.
Hitler used this opportunity to gain favor with the army. He (along with Göring, Goebbels, Himmler and Hess) had some documents forged that would show how the leaders of the SA were plotting to over throw the Nazis with the assistance of France. He justified their executions claiming that they were a treasonous coupe. This act is considered one of turning points in giving Hitler supreme power of the German government.