Inbound links are what it's called when another website displays a link to your website. Search engines then read that link as someone endorsing your site, so the more endorsements to your site the higher your site ranks, and the quicker your site gets updated by search engines when you add new information.
One way to build these inbound links you can contact other website owners and request they link to you, or you can have someone like me do that for you. There is of course more than one way to do it, but this is a simple beginning to explaining what inbound links are.
It is an ongoing and long term process. I still do this for my personal website every couple of days which has been up and running for many years. If you were considering hiring someone to do it, you should look at it as an advertising or marketing expense and budget accordingly.