A backlink is a link on one website that points to another website. When you click on the link, it takes you to the other site. For example, if Site A links to Site B and both sites have great, unique and relevant content, then Site B may rank higher in Google’s index than Sites C and D which don’t have any links pointing at them from high-quality sites like Site A.
Google (and other search engines) use backlinks as a signal of content quality. Take a look at what Duck-Duck-Go says about the importance of backlinks in search engine rankings:
Read more about backlinks here: http://direction.com/backlinks-101/