Syndication packages are an action type available to groups in a network. They show up like other actions, in the actions tab of your group's manage page. They're a great way for networked groups to campaign together in a distributed way.
Syndication allows you to choose an email and/or action as a template and send it to other groups in your network. Receiving groups will be able to quickly edit and publish or send those emails or actions, using your template as a starting point and adding their own branding and customization. As the creator of the syndication package, you can then see which groups have published or sent your syndicated actions or emails and all the statistics that go along with it.
For example, you may have started a national petition about your issue, along with sending an email to your list announcing that petition. You could then syndicate that action and email to other groups in your network, giving them the opportunity to publish your petition and send your email with their name and branding. You can track which groups picked up your action and email and how well they did, allowing you to track the campaign's progress.