In case it isn't quite clear from what the others have said:

A "bottleneck" is more of a concept, it's a point of restriction. Think of a bottle of beer and a mason jar of beer. Tip them both upside down, the bottle will take a while to empty while the mason jar dumps out all at once. Now you understand where the term comes from, and the restriction it implies.

Now lick the beer off the floor, realizing what an idiot you where to pour two entire beers out. :chuckles:

The term is generally used in a negitive sense, as in a "bottleneck" is a bad thing to have.