rel=canonical is a way to indicate that a hyperlink links to the original and canonical version of the current page.
Compare rel=canonical with rel=alternate.
You should use rel=canonical to reduce, minimize, and ideally eliminate any duplicate search results for your pages, i.e. you should canonicalize the following variants of your homepage and permalinks:
Put a link rel=canonical in the head of your homepage and permalinks pages, e.g.:
<link rel=canonical href="https://you.example.com/permalink" />
Tantek Çelik has had rel=canonical support for a while from his homepage and permalinks to their plain versions without any "?" query, and for the permalinks with the explicit URL slug. On 2019-272 he added support for http: pages to link rel-canonical explicitly to the https version of the URL so that Google Search and other search engines would prefer the https versions and avoid duplicate indexing of permalinks.
- Duplicate Content and SEO | Medium