Link building is the ongoing work of earning links from other websites back to yours. Since search engines treat each of those links as a vote of confidence, more good ones can lift how trustworthy and authoritative your site looks, and help it rank.
For a small business, the honest version of this is slower and simpler than the industry makes it sound. It is being listed in the local chamber directory, being mentioned by a supplier or a partner you already work with, sponsoring a community event that names its sponsors, or publishing something genuinely worth citing. Each is a real relationship or a real piece of work, and each earns a link the honest way.
It is worth knowing the other side too: buying links or blasting them from spammy, unrelated sites is against Google’s guidelines and can actively hurt you. Quality and relevance beat quantity every time. A single backlink from a respected local source is worth more than a hundred junk ones. Links are earned off your site, through work and relationships worth pointing to, which is exactly why they cannot be faked from the inside.