Canonical tags are an essential tool to prevent duplicate content in organic search results. “Canonical” in search engine parlance means the one true page out of potentially many duplicates.
Canonical tags are single lines of code in the HTML <head> section of a web page. They are invisible to visitors but not to search bots.
Note the example below from Kohl’s, which assigns a canonical tag for a Bedding and Bath sale page. Details here.