When something disturbing happens it gets stored in the brain in a way that our human system feels that event is either going to happen again at any moment, or is happening now. When some event happens that may be similar or just has an element that reminds the system of that disturbing event, the brain reacts as if the original disturbing event is happening.
EMDR (Eye Movement Desensitization Reprocessing) helps to move the storage of that memory to a more functional part of the brain that can experience the event as actually being in the past. It is important to know that there is a real physical change happening with EMDR. The events that used to trigger the brain into over-reaction no longer have that effect. The person can now react to the present without the past interfering.