mkdir is a Unix command for making a directory, or folder.
Making a directory is very simple from the command line. If you want to make a directory, say for your music, then navigate to your home directory, /home/username, and issue the command
This will create the directory /home/username/music
You can also create directories in other directories that you have write access to, like the /tmp directory, without navigating to it. Just enter the full path of the new directory.