Links from other websites and services, that can direct visitors to your website.
A backlink is created when an external website is linking to your website. This can be both direct links to the home page, a dedicated landing page or be slightly more complex carrying UTM information.
Many search engines see backlinks like a "vote of confidence", if you get good, relevant links from websites with high authority and traffic, it will over time boost your search engine ranking.
Note: The strength of the vote is based on many things, such as click-through rate on the link, the anchor text and relation of the link, the bounce rate when the visitor lands on your site and more.
This effect is also seen in the opposite direction, where linking out, both to high and low authority sites will help both you and them.
It is all about balance, most search engines want to link to real sites with real content, and most real things have a balance of high and low.
Note: While technically all backlinks are good, some are worth significantly more than others. If you for example sell skincare products, getting a backlink from a fashion magazine will benefit you significantly more than one from a car dealership.