Hard-to-engage customers

Can you please explain what is " hard-to-engage customers"?

Multi-channel programs are paramount in the age of social media. Customers are everywhere, and your brand needs to be omnipresent. Marketing automation can help reach these consumers on different platforms, at the right time, without spamming them. Automated emails are just the tip of the iceberg; social interactions and text messages can now all be automated, and are useful for interacting with hard-to-engage customers.