If you have a single page that is accessible by multiple URLs, or if you have multiple pages that have similar or duplicate content, Google will go in and find those pages and choose which one should be the source of information. Canonical links allow you to tell Google which page is the correct source of information.
You may have heard that it’s important not to duplicate content, and usually it isn’t best practice. But sometimes duplicate content is unavoidable — if you’re writing lots of blogs or refreshing content often, Google gives you the choice to tell them which page is preferred with canonical links.