What does dkim=none mean?

Senders insert a digital signature into the message in the DKIM-Signature header, which receivers then verify.

• none = 'The message was not signed'
  This means that the message had no DKIM signature. This is not the same as failing.

https://www.emailonacid.com/blog/article/email-development/what_is_dkim_everything_you_need_to_know_about_digital_signatures