A backlink is a link from one page to another, such as this one! Links are a signal of the relationship between pages, essentially a citation. When someone links back to another page or another website when talking about a certain topic, there’s a high chance the other page/site is relevant to the topic at hand.
This is what the early Google algorithm was based on. If a page had a lot of backlinks pointing at it when talking about a certain topic, the page was probably about said topic and would be surfaced in results when a user searches for the topic. The algorithm has become a lot more nuanced over the years, but links still form an important part of it.