According to en.wikipedia.org/wiki/Backlink backlinks are:
Incoming links to a website or web page. Inbound links were originally important (prior to the emergence of search engines) as a primary means of web navigation; today their significance lies in search engine optimization (SEO).
Backlinks are the single most important factor for getting your website ranked in Google and found by your customers.
You can build backlinks or they can appear naturally. For them to appear naturally, someone must want to link to your site from their own site. So, if you have something valuable on your site that they want their website visitors easily find then they will put a link to your site on their site.
To manually build backlinks means you proactively add your link to other people’s websites.
There are many different ways to do this but one of the easiest is to start commenting on blogs and forums.
Identify the ones that are within your industry and start making useful contributions to discussions on forums or valuable comments on blog posts.
Then, either in your signature or where you can add your website address simply leave your website address and soon you will have built a new link.
It’s important to note that not all links are created equal however, to ensure your link building looks naturally you should identify a wide range of websites to leave your links.
Very basically speaking, the more links you have the better your ranking will be.