A "bottleneck" refers to a feature or function that slows the progress of something. In common language narrow points on paths or stretches of highway with fewer lanes are referred to as bottlenecks or bottleneck points. See the wikipedia article for more.
The term is used in gaming and especially in game economies to refer to objects or tasks that slow down the flow of goods or progress, generally placed there by developers to constrain or control player behavior. Buffs and especially nerfs can function as bottlenecks, if the developers deem the overall flow of events or gold/crafting/experience/etc gain too fast or easy for the overall good of the game or economy.
Bottlenecks are often used as an alternative to nerfing a feature, or as a way to balance a more valuable feature. For instance, Diablo 3 v1.07 introduced numerous new crafting recipes that could create top quality shoulders, amulets, bracers, gloves, or chest armor. However, along with other crafting materials and some gold, a Demonic Essence was required for every such item craft, and the DEs were account bound and thus had to be farmed, which limited the number of crafts players could make. (Which kept rich players from simply buying hundreds of DEs and doing all the crafting they wished.
Many such bottlenecks are present in Reaper of Souls, with gems and other account bound materials required for, and limiting the ease of, most item crafting. In fact, the entire system of character gear progression in Reaper of Souls could be considered a bottleneck, with all legendary and set items (the top quality items in the game) set as BoA and thus forcing players to play the game in order to ever progress their characters in high end power.