Dictionary
Canonical URL
A link tag telling search engines which address is the authoritative one when the same content is reachable at more than one URL.
In more detail
What It Means
A canonical link tells search engines which URL is the real, authoritative one when the same content can be reached at more than one address, such as with and without a trailing slash. Without it, ranking signals can split between duplicate URLs instead of consolidating on one.
See it in context on the homepage: The Head.