This is a term coined by Eliyadu Goldratt in his famous novel The Goal. He another physicist turned business person so I liked his ideas (smile).
He brought clarity to the concept that for any process you can imagine that there is one limiting factor, one bottleneck which limits throughput of the entire system.
An easy way to visualize this a milk bottling plant.
There is one bottleneck which limits the rate at which bottles of milk are manufactured. You can only find that bottleneck by observation - it might be that you only have one machine which can put foil caps on - and it can only work at 100 caps a minute. Maybe it is the amount of raw milk you can enter into the system etc.
Whatever it is, if you don’t identify the bottleneck correctly and put time into improving things which are not the bottleneck then you won’t see any significant improvement in output.