A campaign medal is a military decoration

Military decoration

A military decoration is a decoration given to military personnel or units for heroism in battle or distinguished service. They are designed to be worn on military uniform....

 which is awarded to a member of the military who serves in a designated military operation

Military operation

Military operation is the coordinated military actions of a state in response to a developing situation. These actions are designed as a military plan to resolve the situation in the state's favor. Operations may be of combat or non-combat types, and are referred to by a code name for the purpose...

 or performs duty in a geographical theater. Campaign medals are very similar to service medal

Service medal

A service medal is a military award generally of the lowest degree. A service medal is awarded to a member of the military who joins the military, or is already serving, during a designated time period...

s but carry a higher status as the award usually involves deployment to a foreign region or service in a combat zone.
Campaign medals were first invented to recognize general military service in war, in contrast to meritorious decorations which were only issued on a small scale for acts of heroism and bravery.