A backlink is a link on someone else's website that sends the reader to your site. Think of it as an expert testimonial. If a blogger or someone else from your industry is referring people to your page, they must think it's pretty great.
Google rates pages based on their relevance and link popularity, as well as on their expertise, authoritativeness, and trustworthiness, or E-A-T score. For Google to rank a page highly, it needs to know that other people think its content is valuable, and it does it by - among other things, - taking into consideration how many are linking to your website.