Backlink

A backlink is simply a link from another website that points to yours. If a blog writes an article and links to one of your pages, that link is a backlink for you.

Search engines pay attention to backlinks because they read them as a kind of vote. When many respected sites link to a page, that is a signal the page is worth showing to searchers. So backlinks are one of the things that influence where your pages rank.

Other website  --link-->  Your page  =  a signal of trust

The important word is respected. Not all backlinks carry the same weight. A single link from a well-known, relevant site is worth far more than dozens of links from spammy directories nobody reads. In fact, piles of low-quality links can hurt you, because search engines are good at spotting attempts to game the system.

This is also why buying backlinks in bulk is a bad idea. It looks unnatural, it violates search engine guidelines, and it can get your site penalized. The links that actually help are the ones you earn by publishing something worth linking to.

For beginners, the takeaway is not to chase links directly. It is to make pages good enough that other people want to reference them. To see how this fits the wider picture, read how SEO makes money.