A backlink is a link on another website that points to yours. Search engines read backlinks as a sign that other sites find your pages worth sending people to.
Not every backlink counts the same. A link from a site that is itself well linked, on a page about your topic, is worth far more than a link from an unrelated page nobody reads.
Links also carry a rel attribute that tells search engines how to treat them: none (a normal, dofollow link), nofollow, sponsored or ugc.
Why it matters for a launch
A brand-new domain has no backlinks, which is the main reason it is slow to show up in search. The first handful, from launch platforms, directories and people writing about you, are what get your pages crawled and ranked.
Read more: Free dofollow backlinks for a new site