The mechanism of injury refers to the way damage to skin, muscles, organs and bones happen. Healthcare providers use mechanism of injury to help determine how likely it is that a serious injury has occurred.
A low-speed fender-bender in a parking lot is much less likely to cause a life-threatening injury than a rollover accident on the freeway. A gunshot wound has more potential for serious injury than a fistfight. Tripping over a loose rug can cause an injury. In that case, the healthcare providers are likely to list the mechanism of injury as a ground-level fall, which is not as bad as a "long" fall (falling from a height three times as tall as the patient).