When does bottlenecking happen?

A bottleneck is a prosses in a chain of processes which has such a low capacity, that it slows down the entire chain.

You probably already see that this is a problem you don't want to have, because it limits the performance of the rest. When you use your computer you have to look at it as a chain of processes too. You input (mouse/keyboard) is registered and given to the CPU which the gives the instructions to the other components what to do next. Then the GPU is rendering the next frame for the screen, your visual output. This is an oversimplified explanation, in reality there is happening a lot more in a PC all the time.

If now you have the newest Graphics Card, but only a first gen CPU, the GPU is going to wait most of the time for new instructions from the CPU. Then your CPU is nearly dying performing the tasks while the GPU is always in idle-mode and never using its full potential.

So next time when you think about upgrading you PC, do it by replacing the slowest parts first.