Interaction generally refers to topics, posts or comments. Discussions of some sort.
Some communities include likes, up-votes etc depending on the primary goal of the community.
We're having an interaction right now. Rather than you reading a blog post, you've asked a question and gotten a response.
I wouldn't call a forum a content-centred community. To my mind, forums are about relationships and connections. Social networks are about broadcasting messages.
By 'content' in that post, Rich is referring to blog posts, articles etc. If you provide people with answers to questions in a way that means they don't have to interact, they won't. If you create an environment where asking a question becomes the most important thing, you'll build relationships and connections that keep people coming back.
Does that make sense?
It's all good here. I'm not big on rules.