A backlink is simply a hyperlink coming from a website and pointing to your website. Visitors can click that hyperlink and be directed to your website. Backlinks help your website's search engine rankings.
Backlink building is probably the single most important factor in getting your website to rank well in the search engines today. In order to compete strongly or even dominate the rankings with your website, you need to build as many backlinks as possible that point to it.
It is not enough just to have a lot of backlinks, it is the Quality of backlinks along with the Quantity that help you rank better in Search Engines.