Bottlenecking is a term that gets thrown around wildly excessively. It's not always a helpful word.
It's probably most important to think about having a 'balanced' system.
What bottlenecking means is where one component limits other components' performance.
For instance, if you had an RTX 2080 ti, a 1440p 144hz monitor, and a dual core pentium CPU, the CPU would be a bottleneck. If you had an i9 9900k CPU, an RTX 2080 ti, and a 1080p 60hz monitor, the monitor would be a bottleneck - because you'd have a system capable of 1440p high refresh / 4k gaming but only running it on a cheap monitor, preventing the system from delivering its potential.
In very broad brush terms, the CPU does some of the maths for the game and feeds instructions to the GPU for it to do the rest of the maths. If the CPU is too weak, the GPU will be sitting partially idle while the CPU struggles to get through its workload quickly enough. That's a 'CPU bottleneck' and is usually what people mean when talking about a bottleneck.
Obviously hardware doesn't have unlimited performance, so there will actually always be a 'bottleneck'. That's normal. In gaming you want that to be the GPU (usually called being GPU-bound).
You want a GPU powerful enough to deliver smooth gaming performance at the settings you want, but no more. if it's too weak, you don't get a good gaming experience. if it's too powerful, you wasted a load of cash on performance you're not utilising.
Similarly, you want a CPU that is powerful enough to do its share of work for running your games, feed the GPU, and ideally support a future GPU upgrade or two without being a major bottleneck.
But it's not just about the CPU and GPU 'bottlenecks' - you need a system that is balanced overall. If you buy a cheap or badly designed case, you'll have poor airflow that will not cool the components effectively (which impacts performance and lifespan). Not many people would seriously suggest you buy the cheapest PSU you can find to power your GTX 2080 ti either. You don't need a $400 motherboard. But you want to make sure the mobo you do buy has the features you want, and is of acceptable quality (since it delivers power to the processor, among other things).
So in real terms - you want a CPU like a Ryzen 5 3600 or R7 3700x, or Intel equivalents, 16gb 3000MHz-3600MHz RAM, a half decent mobo, case, and power supply, and some storage to put games on. And for 4k gaming, an RTX 2080 Super or RTX 2080 ti.