You can access the Media Manager through the Control Panel, or by going to Site > Media Manager.
The Media Manager is a very useful for uploading or deleting files in the <joomlaroot>/images/ directory on your web server. You can upload new files, delete existing ones, and create sub-directories. The Media Manager offers two views, thumbnails, and details view. The first will show a much smaller version of the original file (assuming that is an image), the second (details) will show the file name, dimensions, and file size. There is also a handy 'delete' button next to each file.
On the toolbar there are two icons/buttons. The first is a 'delete' button. This is used in connection with the check marks next to each file in the Media Manager display area. After you select a file, you can then click the 'delete' button, and the file will be deleted.
The second button is the 'help' button, which loads the help.joomla.org site. Here information on the Media Manager can be found.