The process of obtaining backlinks from relevant websites while also linking out to relevant sites. This area has evolved and no longer is it the person that has the most links considered the best link builder. The quality of the links matter more than ever.
The first step is developing quality content that in turn will deliver quality backlinks from authority sites that are relevant in your industry. By having many unnatural or irrelevant links, it can lead to penalties. Search engines will see that as deceptive or black hat SEO.