Canonical tag
The canonical tag tells search engines which URL is the authoritative version when the same content is reachable at several addresses. It concentrates ranking signals on the chosen version instead of diluting them across duplicates.
In practice
A typical case: a product page accessible with and without a sort parameter. Without a canonical, each variant competes against the others. The tag goes in the head of every page — including the canonical version itself — as an absolute URL. An inconsistent canonical, pointing to a noindex page for example, creates exactly the confusion it was meant to prevent.