Sensitivity refers to the level at which the ventilator will trigger a breath.
CPAP provides, as the name implies, continuous pressure. This is known as PEEP. PEEP is the background pressure that the ventilator always tries to maintain, whether a patient is breathing in or out.
When a patient tries to breathe, they suck in air, lowering the pressure to below the level of PEEP. This alerts the ventilator that the patient is trying to breathe, and so the ventilator starts supplying more gas to maintain the PEEP.
If PEEP is set at 8mm/Hg, the sensitivity threshold for a breath might be 5, 6, 7, or some other number below 8. If the patient is weak and can't take big breaths, you might have a high sensitivity, because they might not be able to take powerful breaths on their own. If they are young and healthy, the sensitivity might be lower.