A repository (repo) is a list of apps installable from that site, the more repos you install, the longer it will take to check for updates. Therefore, you should only install additional repos if they contain apps you want to use and remove the repo if you uninstall the apps it contains. And repos are like any other website, the owner may eventually shut it down, leaving you with a dead link that gets checked each update. Go to the "Tools - App Catalog" menu in App Manager to manage your repos and see which ones have errors. It is normal for a repo to occasionally have an error as the site may be temporarily unreachable, just watch for repos that always have an error so you can get rid of them.

If you install a testing or devel repo you might get unstable upgrades for packages and could have problems with your system or individual apps. Rare but it could happen, just a warning. Just don't run around installing lots of repos to expand the number of apps, you'll be safer if you only install a repo that contains an app you want and isn't in the default repos.

The three green links at http://asui.garage.maemo.org/download.html are common repos and are safe to install. Most everyone probably has extras-devel installed even though it is a testing repo. I only have the two nokia repos, extras, extras-devel, sdk/tools, community SSU and these two:

If your system won't boot or endlessly reboots then it is time to reflash. Always a good idea to backup to memory card before playing around in control panel and app manager.