Repositories on Ubuntu are the locations that you can download software from. As a general rule, the default repositories don’t contain the right locations for most software packages that you’ll want to install. You will want to open up the /etc/apt/sources.list file, find and uncomment the following lines
deb http://us.archive.ubuntu.com/ubuntu dapper universe main restricted universe
deb http://security.ubuntu.com/ubuntu dapper-security universe
Note that if you are using a different version than Dapper Drake (6.06), you will probably see a different name there, something like breezy or edgy.
Once you are done adding the repositories, you’ll need to run this command:
sudo apt-get update
(Thanks to Gregg in the comments for catching that)