A backlink is simply a link to your site.
It will benefit because a search engine will see the link as a 'vote' for your site from the site that has the link on it.
In its simplest from, the more 'votes' for your site that a search engine can find all around the internet, the higher the search engine will rank your site.
In reality things are much more complicated, as the popularity of the site that is 'voting', the wording of the link anchor text, meta data, the target page on your site and other factors all impact on exactly how each individual 'vote' is valued by the search engine.
I hope this helps,