A backlink is a link from one page back to your page. For example if you had just created a website you would want people to visit it so you'd try to post links on websites to it.
The most common method of back-linking is producing content for websites and then getting a link to it in return. Not all sites allow this. Spamming will hurt you and your site.
Another consideration is that many sites use "no-follow" which means your links will have no effect on page rank so your wasting your time. You can tell if a site uses no follow links by right clicking a link in chrome and selecting "inspect element".