An inbound link is one from someone else’s website to some content on your website.

This is good because if that someone has a a popular website, then prospective customers may follow the link.

From Google’s perspective, the number of these inbound links to your site and the authority (or trustworthiness) of the sites which link back to your site are a good indicator of how trustworthy your site is and therefore how high up Google’s results pages your website should rank.