Dissociation is described as a feeling of being disconnected from yourself, your surroundings, or from reality. People may experience dissociation when they're extremely stressed out or after trauma. It can also be a symptom of a few different mental illnesses like obsessive-compulsive disorder, borderline personality disorder, schizophrenia, PTSD, and others.
For some people who experience dissociation, it feels like an out-of-body experience. They may have an altered sense of passing time, feel a sense of emotional numbness, or feel like they are a completely different person.
Dissociation can sometimes be scary, but for many people, it passes on its own, and doesn't necessarily mean that you have a mental illness. If you experience dissociation frequently, though, it could be a sign of something more serious.