I am going to add the answer as it is in Official wiki
Data on a computer, as you may know, is stored in binary as a series of 1s and 0s. The way these are stored on a device and their structure is called the "filesystem". In Linux devices are referenced in /dev. Data is not actually stored on a device so you cannot access this data by going into /dev, this is because it is stored inside the filesystem on the device so you need to access these filesystems somehow. Accessing such filesystems is called "mounting" them, and in Linux (like any UNIX system) you can mount filesystems into any directory, that is, make the files stored in that filesystem accessible when you go into a certain directory. These directories are called the "mount points" of a filesystem. In other systems this is done differently. For example in Windows there is no distinction made between a device and the filesystem on it, and the user is restricted to mounting a device's filesystem in a top-level volume which is automatically assigned a letter such as C:, D:, etc. and the files inside these filesystems are accessed inside each volume's root such as "C:\", "D:\", "E:\", etc. (remember, Windows uses back slashes instead of the more common forward slashes you find in Linux)