A court Marshall is where members of the military such as military personal that are in the military go against the court of the military for a crime or infraction that they did against the military.
The court martial is empowered to determine the guilt of members of the armed forces subject to military law, and, if the defendant is found guilty, to decide upon punishment.
Being court Marshall is never a good thing and if the crime you did was serious enough you could go to military prison or even spend life in prison.
When in the military you want to avoid being brought up against the court martial.
Basically the court martial is a court of law for the military operated by the military and similar to a regular court of law.