A Backlink is a link which is connecting one website page to the page of any other site.
Having said that I basically mean that when you get a link to your website from any other website, you receive a backlink. Links that are coming from any external sites to your site are known as Inbound links (IBL’s) or incoming links.
But one should be very careful while linking their website with any other website as there are Good links and Bad links.
If you get an increase in your ranking by linking your site from a Good link then You may also get a decrease in your rankings and hurt your domain authority if you get a link from any Bad Link.
Please refer to the link to know more about Good Links & Bad Links.