By backlink definition, it is a link on one website for another website. Consider backlink as a vote, more votes (backlinks) you will get from other websites, more authority you will get and your blog will get better ranking in search engines.
Backlink SEO is a practice to get better ranks in search engine and many online seo companies sell backlinks which is not a good idea to buy.
To get better ranking, always try to get natural backlinks and make sure you are getting backlinks from websites relavant to your own blog’s niche. Getting a backlink from a restaurant website if you are running a technology blog will not work and you may get even penalized for this.