All files in a Linux filesystem are arranged in form of a big tree rooted at ‘/‘.These files can be spread out on various devices based on your partition table, initially your parent directory is mounted(i.e attached) to this tree at ‘/‘, others can be mounted manually using GUI interface(if available) or using mount command.

mount command is used to mount the filesystem found on a device to big tree structure(Linux filesystem) rooted at ‘/‘. Conversely, another command umount can be used to detach these devices from the Tree.