it means that other websites links to your site without the need to link back to them from the same site
Backlinks are the lifeblood of Search Engine Positioning. In order for a website to be on the top of search engines like Google without the webmaster having to pay big money for advertising, the website has to have a large number of backlinks. Backlinks are links on one website that lead back to another website. If you have more and more backlinks the more likely you will can increase page rank, the higher the page rank (1-10 scale) the more important. (Please note page rank is not always directly proportional to improve SERP)
backlink? as far as i concern backlinks mean that link of your website is being put to other website...or appear on another website..the more backlinks you have..the higher PR you would get...and will generate more traffic to your website slowly...correct me if I'm wrong
Guess You have a site "A" and "B" is another site,so if the site B links up with your site (I mean the site "A"),then you are getting 1 backlink.It means one site is representing your site and in the eye of Google search engine it means a lot for your site to be ranked up.The more you get backlink you will be ranked up.
building backlinks is building links for your site. reason its called backlink is because it is an incoming link form a 3rd party site. when you visit a blog for example and see few links on the blogroll linking to some external sites, those are backlinks fro the sites where the links are pointing to for example. google considers the backlinks as votes and more backlinks yor site have, more vote you got and higher your site ranks in general. there are many variables that need to be taken into accounts but thats the common conception