Before we start talking about bottleneck in computer, let's start with what the term "bottleneck" means.
A bottleneck is when the performance of a system is blocked / limited by a single component of the system.
To relate this to computers, a bottleneck is when one of your parts (CPU, GPU, RAM, Hard drive) are holding the rest of the computer back since they aren't performing good enough to keep up with the other parts of your computer.
In daily life conditions, hard drives are the components which mostly cause bottlenecks. For example when you are copying a file, no matter how fast your CPU is and no matter how much RAM you have, you are...
