Link building in 2018 is the process of earning links on other websites. Earned natural links directly improve the reputation of a website and where it ranks in Google, and other search engines. Self-made links are risky and come with them the risk of penalisation by the Google webspam team.
Link building can be a very risky strategy in 2018, and one that I would avoid until you have (at least) first dealt with the inevitable quality issues on your website. If your business or website content is not attracting organic links or social shares you may have a big problem on your hands, and unnatural links will not help you in the long run.