What does bottleneck effect mean?

Well, think about it. Imagine the same amount of fluid in two different containers, one bottle, and one jam jar. Think about which will empty fastest if you turn them upside down. Bottleneck refers to anything that restricts flow. If you have a two-lane road that narrows to one lane, that's a bottleneck. If you have a walkway in a park that pass through a turnstile, that's a bottleneck too.