A backlink is a link coming from another website to your own. The number and quality of backlinks that your site has can affect your search engine optimization efforts, as some search engines provide significant weight to the backlinks of a site.
If you are working on an SEO campaign, you should include efforts to get links to your site on related sites with high ranking in search engines for the terms you are targeting. This "link love" will help improve your search engine ranking as well. Try to avoid being linked on link farms as that can hurt your search engine results.