Canonical Tag
The canonical tag tells search engines which version of a page, among several similar ones, should be treated as the reference.
The same content accessible through several URLs, with or without parameters, over HTTP or HTTPS, with or without www, creates a duplicate content risk. The canonical tag settles this by pointing to one reference URL.
Without it, a page's authority can get split across several competing versions, and none ranks as well as a single, consolidated page would.