Backlinking -> getting links to your site. Lets say you have a site which sells flower-pots. You get a link to your site from a forum, blog, social network, etc etc, which has SOME relation to flowers etc.
So it is linking content related sites together.
I have a page for punching bags therefore I will write an article on the subject of using punching bags for a workout, and link that article to my page. Thats a backlink. You need a lot of them is the competition for your keyword is high.