Canonical tag
A canonical tag is a line in a page's code telling Google which version of a page is the real one when several URLs show the same content.
Shops generate this constantly, where the same product sits at 5 URLs because of filters, sort orders and tracking parameters.
Without the tag, Google picks one itself, and it may not pick the one you want ranking.
It's a strong hint rather than an instruction, so Google can and sometimes does ignore it.
The reliable version is to have 1 URL per piece of content in the first place and use the tag to mop up what the platform generates.
A canonical pointing at a page that redirects, or at a page blocked from indexing, sends a contradictory signal and gets disregarded.
What this means for you
This is plumbing, and it's the kind of plumbing that quietly splits the strength of 1 good page across 5 weak copies of it.
This one sits under SEO.
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