A playlist is a collection of MP3s from the database.
To view the content of an playlist simply click on the playlists' list (on the left side of the screen)
You can create a playlist by clicking on the "New playlist" button
To add songs to a playlist you can select them and drag them to the playlist box or you can use the right click menu and click on the playlist from the "Insert in playlist" submenu.
You can also create playlist folders by clicking on "New folders". To create a playlist into a folder, select the folder you want to be the parent of that playlist.
You can also drag & drop playlists into folders.