Sensitivity is a measurement of the amount of change in a camera's field of view that qualifies as potential motion detection,.
Threshold is how much of that motion needs to occur in order to actually trigger the alarm.
For however long there is enough change to satisfy the sensitivity parameter, a threshold meter is silently being filled up until it hit's the trigger point.
Considering this, if you set your sensitivity to 1 and threshold to 100, it would be practically impossible to trigger a motion alert. Alternatively, if you set your sensitivity to 100 and your threshold to 1 or 0, this means that the slightest change could set off the trigger. The trick is to finding a healthy balance based on all the factors that stand to stimulate your motion detection sensors and doing your own personal testing."