From IFWiki

A bottleneck is a situation that must be passed through to reach new content, in which options are dramatically reduced; the term can be applied both to a branching plot and to the map. In many IF games, physical and narrative bottlenecks are the same thing; sometimes a bottleneck will be a literal door or similar.

Bottlenecks give the author a higher degree of control over pacing and plot, limit combinatorial explosion and provide an obvious goal with a tangible reward (access to content on the other side). On the other hand, their overuse may make a game excessively linear.