Pressure in a circuit built to stay low
The pulmonary circulation normally runs at far lower pressure than the systemic circulation, because the right ventricle is a thin-walled pump suited to a low-resistance bed. Pulmonary hypertension means the mean arterial pressure in that bed is persistently raised above the normal range, forcing the right heart to work against resistance it was not built for.
Sustained overload causes the right ventricle to thicken and then dilate. Over time it can fail, which is why the condition is followed by how well the right heart copes, not by the pressure number alone.
Many roads to one pattern
The raised pressure can come from disease of the small pulmonary arteries themselves (pulmonary arterial hypertension), from chronic clots, from left heart disease backing up into the lungs, from chronic lung disease and low oxygen, or from mixed causes. Clinicians group these because cause guides management, and treatments for one group can be unhelpful in another.
This is general educational information about the physiology. Pulmonary hypertension is diagnosed and classified with specialist testing and should be managed by a qualified clinician.