The Plesk Panel is a product of Parallels that provides you with a web interface to manage your 1&1 Server. The Plesk Panel differs from the 1&1 Control Panel in that Plesk is used to assist you with server configuration that would normally be done via SSH (Secure Shell) or Remote Desktop.
Instead, you can access the Plesk URL, log in, and navigate the interface with clickable buttons to perform tasks such as managing databases, E-mail accounts, webmail interface, etc. The Plesk software is included in the price of your 1&1 Server, however it is not mandatory to use Plesk with your 1&1 Server. Some customers choose to handle their server administration via the command line or remote desktop.
It is possible to try a demo of Plesk from the Parallels Plesk Demo Page in order to try the software before deciding whether you wish to utilise it for administration of your 1&1 Server.