A bottleneck is the point in a process where capacity narrows and everything behind it queues, named for the neck of a bottle, which sets how fast the whole bottle pours no matter how wide the base is. In corporate speech it covers anything that limits throughput: an approval that sits with one person, a meeting room everyone needs, a lift lobby at nine oclock.
The useful discipline behind the jargon comes from the theory of constraints: a system moves at the speed of its tightest point, so improving anything except the bottleneck improves nothing. Workplaces are full of physical ones, and utilisation data is how you find them: the one room that is always booked, the check-in queue at reception, the car park that fills by 8:40. Fix the constraint, then find the next one, because there is always a next one.