After you mount such a device you can access the files and directories stored on it in the same way as the files and directories on the internal hard drive.
The command is call "mount" since it was used originally to complete the process of placing a magnetic tape on a drive and establishing a data connection from the tape drive to the computer. The command creates a new directory (folder) in the directory tree of the computer's file system. The "mount point" defines where exactly in the directory tree that is.
Modern computer systems automatically perform most mount operations when a new device is connected, so that the user doesn't have to explicitly type in the command or initiate it through clicking an icon. However, it is still recommended to explicitly unmount a device before disconnecting it, so that the operating system can make sure all data have been properly saved.
Mount related links: