A bottleneck is the place where transmission is the slowest.
A bottleneck can best be described as the slowest working component in a machine. Suggest you have a computer with 100 super fast CPU's, a bit of memory and an old IDE drive. When we start a process that needs to do parse something from a giant file, our bottleneck will be the I/O. Our CPU's will be able to handle the parsing, but not all of them will be used on max capacity because the I/O will not be able to provide the file fast enough.
This is called a bottleneck. The analogy comes from whenever you try to empty a bottle. There is only so much liquid that can get trough the bottle opening (bottleneck).