When launching a new Lemmy instance, your All feed will have very little populated. Also as a small instance, new communities that crop up may never make their way to you. LCS is a tool to seed communities, so your users have something in their All feed, right from the start. It tells your instance to pull the top communities and the communities with the top posts from your favorite instances (and subscribe to them with the specified user).
Latest documentation available at: https://nowsci.com/lemmy/lcs/