A trauma is any event that is scary, somewhat unusual, and at least potentially dangerous.
This could include (not a complete list):
A natural disaster, like a fire, flood, or tornado
Abuse, including physical abuse, sexual abuse, or domestic violence
Witnessing a violent death, from accident, suicide, or homicide
A serious accident, usually much more severe than a fender bender
A serious medical procedure, especially if unplanned
Being involved in a war, as a participant or bystander
Our bodies are made to process a certain amount of stress. Different people can handle different amounts of stress. Traumas at least temporarily can overwhelm our ability to process the stress. If the stress starts interfering in our ability to do the things we want to do or to manage our feelings and relationships, it may be time to ask for help.
We cope with "micro-traumas" every day in the form of annoyances, more everyday problems, hurtful relationships, etc. Most of the time these only cause brief distress. The more resilient (able to bounce back) we are, the more of these distressing circumstances we can handle. Good self-care and maintaining healthy relationships can help us be more resilient. Sometimes, though, the cumulative trauma gets to be too much for us. Think of an in box with items coming in so quickly that you can't move them fast enough to the out box, and the box starts to overflow. This can also be a reason for seeking a professional to talk to.