An inbound link is a link from an external domain pointing towards one of your pages. For example, if you ran example.com, and HubSpot.com had a link to your site, then that would count as one inbound link.
Google uses inbound links as one of its main sources for gauging the quality of your content. Having inbound links from sites with information relevant to your site's main focus will help increase your SEO as well as referral traffic to your site.
See your topic cluster inbound links
Your content strategy tool will show you the aggregate number of inbound links to all pieces of content within a topic cluster. You can access this information by navigating to Content > Strategy.
See the quality of your inbound links
You can also gain some insight into the quality of these inbound links. Inbound links from authoritative and relevant sites are considered of higher quality than sites with information that's not relevant to your core topic. If any low-quality inbound links are present on your subtopic content, you'll see an in-app message that reads Low quality inbound links in the subtopic details on the right sidebar.
For tips on how to get more inbound links, check out this blog post which provides tips on getting sites to link to your content.