It could mean any number of things. First and foremost, serious trauma requires immediate surgery, and quite often surgeons with different specialties. A single patient might need a thoracic surgeon, a neurosurgeon and an orthopedic surgeon (they might need more than that but you get the idea).
In addition to the surgeons, they'll also need a surgical team, an available operating room, anesthesiology, radiology with CT capabilities, blood bank, and a bed in ICU. Most smaller hospitals aren't likely to have multiple surgeons and all the support staff and facilities immediately available. It's extremely expensive to maintain such resources 24/7, especially since they would be idle much of the time at a small hospital.
This is why trauma systems were developed in the US. These are hospitals specially designated as trauma centers. Typically, they're large, urban hospitals that have the patient volume and financial resources to be able to afford keeping such staff and facilities available. Along with the trauma centers, regulations exist that define which patients meet the criteria of being "trauma patients" and EMS will transport all such patients to the nearest trauma center rather than simply the nearest hospital.
See also my answer to this question.