A bottleneck is a place where flow slows down or stops because everything has to squeeze through a narrow opening. Picture trying to pour water quickly from a wide bucket into a narrow-necked bottle: no matter how fast you pour, the water can only pass through that small opening at a certain speed. The bottle's neck limits the flow, creating a backup.

The word comes from this literal image but applies to many situations. In traffic, a bottleneck occurs where a four-lane highway narrows to two lanes, causing cars to bunch up and slow down even if the road ahead is clear. In a factory, one slow machine can create a bottleneck that holds up the entire production line. At school, if everyone tries to leave through one exit at dismissal, that doorway becomes a bottleneck.

The concept matters because identifying bottlenecks helps solve problems. In a game, if one teammate has trouble keeping up, that situation can be a bottleneck for the team's performance. Once you spot the bottleneck, you can work on solutions: opening another exit, adding more machines, or helping that teammate get extra practice. The key insight is that speeding up everything else won't help until you fix the actual bottleneck.