A backlink is a link from someone else's website to yours. Search engines read it as a vote of confidence: a site that others reference is presumed more trustworthy. Quality dominates quantity — one link from a respected industry site outweighs hundreds from directories nobody reads, and paid link schemes are penalised.
For AI visibility, mentions matter even without the link: engines cross-reference entities across the web. The honest playbook is in link building that still works.