Everyone has to ability to slightly dissasociate when needed and this is usually in a mild way and is temporary. It is usually controlable and by choice. When dissacociation is more extreme it can cause issues and depending on the level would depend on how the medical profession would describe it.
Sometimes people feel out of sorts, disconnected to themselves, their bodies or the world around them and sometimes a person can be just dissacociated from their own thoughts or feelings. If this affects a person functionning in every day life then this can become a more significant problem.
There are certain psychological conditions and disorders which has dissacociation as part of the conditions such as Personality Disorders and past Trauma of some kind.
The process of therapy aims to work with the particular type of problem going on and integrate either memories, personality parts, thoughts, feelings and experiences in a way that is helpful and promoting of internal and external balance.