A source, defined by dictionary.com is
One, such as a person or document, that supplies information...The same definition applies to the jailbroken term source. A source is a website that hosts packages like games, themes, and other files for Cydia to download. When you type in this website into a computer, you will get a not-so-great looking page but when you type the site into Cydia and install the source, (Cydia then Manage then Sources then Edit then Add) this page gives the information Cydia needs to download more packages from that source.
The following sources can be manually installed in Cydia by going to Manage>Sources>Edit>Add and typing them in. The more sources you have, the longer Cydia will take to load but you will have a wider choice of downloading themes, games, and other packages.
Sources that are in green are those that should already be installed in Cydia, they are known as Community Sources. These are the sources that have most of the packages.
BigBoss & Planet-iPhones: http://apt.bigboss.us.com/repofiles/cydia/
Niklas Schroder: http://apt.paperclipsandscrambledeggs.com/
Ste Packaging: http://repo.smxy.org/cydia/apt/
Steffwiz: Now part of PwnCenter Source
Telesphoreo Tangelo: http://apt.saurik.com/