Cold outreach means emailing people who don't know you. In link building, that usually means writing to the editor or author of a post and asking them to consider adding your product or resource.
It works when each email is clearly written for that person: it names their post, the section where you'd fit and why their readers would care. It fails when it's a mass message with a name swapped in.
Cold outreach also carries deliverability risk, because some recipients will mark even good emails as spam. That's why it's sent from a separate domain, at low daily volume, with proper authentication.
Example
Margin emailing a blogger it has never spoken to, to suggest adding Margin to their list of research apps, is cold outreach.
Putting it into practice
Where to use it: Listicle inclusion email template, Link outreach email checker and Email deliverability for link outreach. For the longer version, read the full link building outreach process.