Canonical tag
A canonical tag tells search engines which URL is the preferred version of a page when the same content is reachable at several addresses.
In full
A canonical tag is a link element in a page's head that names the preferred URL for that content. It exists because the same page is frequently reachable at multiple addresses: with and without a trailing slash, with tracking parameters attached, over http and https, or through a printer-friendly variant. Left unresolved, these compete with each other and split the ranking signals that should accumulate to one address. The canonical tag consolidates them. It is a strong hint rather than a directive, so search engines may occasionally choose a different canonical if the evidence points elsewhere.
Why it matters
Duplicate URLs are one of the most common and least visible technical problems on commercial websites, particularly e-commerce sites with filtering and sorting parameters.