"Red coats" or "redcoats" is a term used for British infantry soldiers during the Napoleonic era. Many of the regiments wore a red or scarlet coatee as part of their uniform and, thus, derived the nickname "redcoats."
New Model Army
In 1645, the English Parliament passed an ordinance to create a professional army, the New Model Army under Oliver Cromwell. Infantry regiments were outfitted with red coats. In colonial warfare, officers preferred that soldiers wear highly visible colours in order to keep troops together during battle.
In the 18th century, the British infantry became the most professional and feared army in the world, and they took pride in their status as "red coats." However, as warfare evolved, camouflaged colours became much safer to wear during battle, and the British infantry last wore red tunics at the Battle of Gennis (December 30, 1885).
Red coats today
Today, the term "red coat" is largely ceremonial. It is dangerous to wear bright colours during battle in contemporary warfare, and the modern British infantry uniforms range in the standard khaki and green colours. Some regiments will wear red or scarlet in certain ceremonial contexts to pay homage to their history.