Allow me to explain:

In order to understand how gaming bottlenecks work we must first understand the roles played by the hardware involved.

The CPU:

The CPU is what calculates positioning, AI and a large number of other factors in a game engine. Every frame drawn must first be calculated by the CPU

The GPU:

The GPU takes the frame details provided by the CPU and renders them on the screen for you to see, it also adds all the visual bling as it does this. 

So when we look at a game we shouldn't be looking at it like a smooth fluid motion centric experience but rather more like an animation where each frame is first laid out by the CPU then rendered and detailed by the GPU.

So how does a bottleneck happen?

We open a game and set it to 720p on low settings, our GPU is able to push 200+ fps at these settings. This means the CPU has to calculate all the data needed to draw 200 frames in a single second. If the CPU is unable to do this we will see the GPU usage go down and framerates drop. Now increasing the settings to 4k on ultra settings we see our GPU only able to push 50fps, the CPU now only has to provide 50 frames worth of data to the GPU per second. 720p or 4k make no difference to the CPU, the data is exactly the same, the load on the CPU is no different. A faster GPU will demand more frame data from the CPU and a slower one less, when we see a point where the CPU is able to provide 200fps worth of data but the GPU is only able to render 50fps we are now seeing a GPU bottleneck. 

In a nutshell this means more fps = more work for the CPU

This is of course a vast oversimplification but the principals are accurate and illustrate exactly what a gaming bottleneck is caused by. 

As we see in my post above, CPUs need to prepare frames to be rendered and the GPU then makes the frames look pretty. So when you are looking to build a new gaming machine the first thing you need to decide is budget, are you going for the best of the best or simply trying to get the best bang for your buck?

Once you have this ironed out we come to part picking.

The first thing you need to look at is a GPU, this you should choose based on cost and performance reviews at the resolution you will be gaming at. The resolution is a very important detail so don't overlook this. You will want to get the best GPU you can within your budget while leaving enough money to purchase the other parts.

So for the sake of this example we choose a GPU that is shown to provide 150fps in most games @1080p because we plan to use a 144hz 1080p monitor.

Now we move on to the CPU, again we need to do our research but rather than going into that blind we now know that we want a CPU capable of providing at least 150fps @1080p. Now picking one that only just manages the task would be a mistake, CPU bottlenecks result in ugly and often inconvenient stuttering and screen freezes while the CPU tries to catch up to the demands of the GPU. So what we really want here is a CPU that is able to provide 150fps @720p or 200fps at 1080p, this gives us some headroom for either more demanding games or future GPU upgrades. The goal here is to pick the CPU that delivers the closest performance to these numbers possible at the best possible price point.

Now we have picked the core of the system we move on to the rest of the system, the only thing I will mention here is making sure you pick a solid, brand name PSU that is about 100w more than the recommended wattage for the CPU and GPU you chose but not less than 500w. The rest of the parts will be up to you and your particular needs for the system outside of gaming.

Following these basic guidelines will allow you to get the absolute most for your money in terms of gaming performance without running into immediate bottlenecks with your shiny new hardware.

simplest:

the place, thing or item that constricts flow of data, things or ideas!