If youre wondering what a back link is, well it is a link placed on another site with content relevant to yours that is directed towards your website. Links to your website direct traffic from the linking point to your site and help you to rank higher in search engines.
Lets say that you have a website that sells grills. To get the most effective links to your site you would want to find other sites that offer content similar to grills to place your link on. These other sites might sell books on grilling, offer recipes for grilling or grill parts. The linking site will ideally have content that relates to grilling in some way.
You should view the internet as a popularity contest and each little link is a vote for your websites quality. Links of quality are much more effective than links in quantity. 10 links from websites with content related to that on your site could be more valuable than 1000 links from unrelated sites.
The links placement on the relevant site is also important. You are better off with a link on the front page of a website than a page that is deep within the site. Ideally you should be able to get to the page your link is placed on by...