An inbound link is a link from one site to another site. If your company's blog included a link to an article on the HubSpot blog, HubSpot would be receiving an inbound link from your site.
In HubSpot, an inbound link is a link to a page on your site from another site that has generated at least one visit. You can see how inbound links are generating traffic to your website pages in the sources report. Traffic to your website from inbound links will be reflected in the Referrals bucket of the sources report (Reports > Analytics Tools > Traffic Analytics > Sources).