“Bottleneck” is a name for any device or interconnect between devices, that has “the slower speed”, thus reducing the speed with which other devices, connected to this “bottleneck” are able to do their thing.
See this image? Looks a little like the neck of a bottle doesn’t it? Imagine that one device, call it “top-half”, wants to send data to another device, which we call “bottom-half”. To do so, it needs to send the data through a port, bus, interface, device, connection, whatever you want to call it. We’ll call it “the pipe”.
You can see, that no matter how much data top-half has, it will only reach bottom-half with the speed, or bandwidth, or gigatransfers of the pipe. So no matter how many Ghz, Gflops, or terabytes the top-half can crunch, it will only send data to the bottom-half at the speed op the pipe, so the bottom-half can only operate at the speed of the pipe, because it simply doesn’t get any more data to process. This is called a “bottleneck”.
So in short, slow RAM isn’t optimal since it can handle less data throughput, a mind-boggling CPU will sit idle for most of the time if the other components and interconnects are slow, and there’s little use in getting a fast SSD if your motherboard can’t handle the speed.
So, the more bottlenecks you find and remove out of your system, the faster it gets. This is one of the magic things that makes the difference between just spending a lot of money, and building a good PC.