How do we build the "Direct" source of organic traffic?

I've been looking at a few successful niche sites and I've noticed something puzzling. Many of them have a huge chunk of their organic traffic coming from the "Direct" source, sometimes even more than the traffic from google search. Does this mean the regular users of the site bookmaked or stored the link to the pages of the site elsewhere for quick access? What are some ways we can encourage this, so that if the algorithm decides to nuke our site, we still have our loyal users/visitors? Sorry if this is a noob question, I'm new to SEO.