Eli Myers (via email) says:
Content Suggestions are topics that we've seen used on pages that are ranking well for the keyword that you queried. If you're looking to help a particular piece of content rank well, they are topics that you may consider including. The idea is that the topic content may be helping those pages rank if they're a relevant and useful.
For example, here is a report from the keyword "seo":
Pages who rank well for "seo" are also talking about '"search console", "seo keywords", "seo agency", among other things. I can hit the arrow on the right to expand the box and see which exact pages on the SERP for "seo" are talking about "seo keywords". This area can be used as ideas for content generation - for example, in a new blog article on my SEO site I might want to talk about Google's Search Console, keyword research, agency topics, etc, as it would seem that searchers are finding this topics relevant and useful.
You can read more about Page Optimization and Content Suggestions here: https://moz.com/help/guides/moz-pro-overview/on-page-optimization
