It seems there's not a lot of helpful newbie style info on what repositories are. I agree that a guide would be a good thing, if only to explain why it's good to use them instead of random binaries.

As a quick answer, repositories are a place where you can get applications (and updates!) for a Linux computer. You tell Linux how to find the repository (the URL). Now the application manager will check what applications are available from the repository, and you can install programs using the application manager. Most repositories take a great deal of pride in making sure that their software is safe, secure, and functional and regularly update the programs with security fixes and the like. This means that programs in a popular/trusted repository won't contain viruses. A repository also provides updates, so serious bugs can be fixed and sent through updates.
The main repository that Maemo users should be familiar with is extras, which contains most of the community apps that have been thoroughly tested.