<link rel="canonical">
Declares which address is the official one when the same page answers to several.
The same page tends to exist at many addresses: with and without a trailing slash, with campaign parameters, over http and https. canonical points at the preferred address, and search engines consolidate there the signals the variants would scatter.
The myth is treating it as an order: it is a strong suggestion, which a search engine may ignore when the rest of the site contradicts it — a sitemap pointing at another variant, redirects in disagreement. Always point at an absolute, living URL; self-reference, on the official page itself, is common and harmless practice.
<link rel="canonical" href="https://exemplo.org/artigo/">