In Christianity, adiaphora means that something is debatable, spiritually neutral. There are essentials of the Christian faith such as the deity of Christ, monotheism, Christ's physical resurrection, etc. But there are also topics that deal with issues that are non-essentials. So in a general sense, adiaphora means those Christian teachings which are neutral, things that are neither commanded nor forbidden in Scripture. An example of this might be what color of carpet to have in a church or what time of the day a service should be held.
Sometimes there is debate within Christianity on what may or may not be adiaphora.