Trauma is an event that has had a profound impact on oneself. For some people this trauma takes time to process. Trauma can be anything from an ending of a relationship to long term abuse.
Trauma is about the type of event that has occurred and the impact it has on the individual. For example some people may not consider a break up traumatic and may think of it one of those things that happens but to others it can be devastating.
Eventually if trauma is not processed it puts your emotional well being at risk. In my experience disconnection is one of the most common aspects of unresolved trauma. Other common feelings include depression, anxiety, feeling overwhelmed and nightmares/flashbacks.
Often some people only realise an event is traumatic when they begin to explore it in therapy. When something is very painful the psyche can build a brick wall around it to prevent it from causing more distress. For this reason some parts of therapy can be very painful because it is a slow breaking of that brick wall that then enables the trauma to be processed as opposed to be locked away and unconsciously effecting a persons behaviour.