Ribbon bars are small decorations that are worn by military

Military

A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...

, police

Police

The police is a personification of the state designated to put in practice the enforced law, protect property and reduce civil disorder in civilian matters. Their powers include the legitimized use of force...

, and fire service personnel or by civilian

Civilian

A civilian under international humanitarian law is a person who is not a member of his or her country's armed forces or other militia. Civilians are distinct from combatants. They are afforded a degree of legal protection from the effects of war and military occupation...

s. Ribbon bars are mainly used when wearing either full decoration, or when wearing medal

Medal

A medal, or medallion, is generally a circular object that has been sculpted, molded, cast, struck, stamped, or some way rendered with an insignia, portrait, or other artistic rendering. A medal may be awarded to a person or organization as a form of recognition for athletic, military, scientific,...

s is considered inappropriate or impractical. Each military force has its own rules on what ribbons are worn in which order of precedence. Some awards are "ribbon only", having no associated medal.
The ribbon bar for a particular medal usually matches the ribbon on the medal.