Engagement encompasses all the activity around your website. For example, if you build a website and it's SEO'd very well -- and it has no visitors and no lings from anywhere else, it won't rank for anything. However, if you have social media channels that people visit your website from, or if your website is being shared a lot by other peoples social media channels, you'll start to rank quite well, almost immediately. The trick is to cross promote your social media accounts and retain the traffic by building an email list. Always ask the visitors/members to engage. The more buzz there is around your online presence, the better your pages will rank. But the most important thing is to create genuinely useful content, products or services which will automatically increase engagement. Create content that people will naturally want to refer to or link to.