A bottleneck is a type of traffic jam, when cars get stuck or slowed down in a narrow part of the roadway. You might leave work two hours later than usual to avoid a bottleneck.
Anything that gets stalled in a narrow area, whether it's auto traffic or pedestrians, can be called a bottleneck. Information can also get stuck in a kind of bottleneck, if the system itself creates a delay. The term comes from the shape of an actual bottleneck, or the narrow neck or mouth of a bottle.