What does it mean when someone subscribes to your public updates on Facebook?

[Lulu]

It means he has subscribed to your PUBLIC updates. So, if you change your overall privacy settings so that none of your posts are Public, or set the settings to be something other than Public for specific posts then he won't see those.

You know the box where you type a new post has a drop down in the lower right corner that lets you customize the audience for the post - Public, Friends, Custom, or one of the groups you make like co-workers, school friends, neighbors, etc.