A military uniform is a set of clothes worn by people in the armed forces. A uniform makes it easy to see that a person is in the forces, and which side they are on. A uniform may be made to impress people, or to frighten them.
In Hannibal's army there were soldiers who wore white tunics with crimson edgings. The Roman armies had uniforms. Their commanders had white cloaks and feathers on their helmets. The centurions had special helmets, and chest ornaments a bit like modern medals [1]
The normal 18th century military uniform in Europe was a three sided hat, long-skirted coat, waistcoat and breeches. Uniforms got more colourful after the Napoleonic Wars. British soldiers had red clothing and were called "Redcoats". By the time of the First World War most uniforms were much duller, and less expensive, because the armies were very large. Grey, khaki and blue were the common colours. Steel helmets became very common.
By World War II plain and practical combat dress had replaced the colourful uniforms, except for parades. Now almost all armies wear some sort of camouflage.[2]
There may be different uniforms for different activities. There may be a dress code. For actual fighting there may now be camouflage. In the past there was often armour. People who have won medals or awards may put them on their uniform or be given special uniform for a parade.