The clipboard is a storage area where your copied data (text, files, etc) are stored temporarily, which can be then retrieved (to paste them, for example, into a text editor, folder, etc) by another program.
To be able to manage your clipboard under Ubuntu, you can try Diodon, which is compatible with the GNOME desktop. To install Diodon on Ubuntu 11.04/10.10/10.04, launch the Terminal and issue these commands:
sudo add-apt-repository ppa:diodon-team/stable
sudo apt-get update
sudo apt-get install diodon
Once launched, the Diodon's icon will appear in the notification area, click it to start managing your clipboard.