What does it mean if a high pressure alarm sounds on a mechanical ventilator?

Occlusion. The high pressure alarm means there is an occlusion somewhere along the system of tubes that lead to the air sacs in the lungs. High pressure can be caused by the patient biting the endotracheal tube, secretions plugging the tube or the airways, bronchspasm, and many other causes.
Usually Obstruction. Anything causing resistance to the ventilator "giving" a breath can cause the alarm. Usually this is from secretions or plugs in the ventilator tubing, tracheostomy or endotracheal tube, or in the patient's lung "air tubes" ( pneumonia, pulmonary edema ). Coughs or resistance to the breath - alarm. A "stiff" (ARDS) or compressed ( blood or air in chest ) lung or high abdominal pressures - alarm.
Possibilities... The ventilator's high pressure alarm sounds when the machine has to deliver the breath at a force that exceeds the highest pressure that the machine has been programmed to accept. There are many reasons for this to occur. For example, there could be an obstruction in the ventilator circuit or the patient could be coughing or wheezing as the vent attempts to deliver the breath.