There are a very small number of resources (men, machines, or materials), that dictate the output of the entire system. It is critical to create an optimized and trusted flow of work to the constraint, otherwise it will be underutilized, thereby limiting the output of the entire system.
Improving only the bottleneck matters, because the whole will always wait for it anyway.
The constraint is the bottleneck.
Defined by Dr. Eliyahu M. Goldratt in The Goal.