Metaphorically a bottleneck is a section of a route with a carrying capacity substantially below that characterising other sections of the same route.
This is often a narrow part of a road, perhaps also with a smaller number of lanes, or a reduction of the number of tracks of a railway line.
It may be due to a narrow bridge or tunnel, a deep cutting or narrow embankment, or work in progress on part of the road or railway.
More generally, a bottleneck is one process in a chain of processes, such that its limited capacity reduces the capacity of the whole chain.
See also Performance problem, Population bottleneck.