"New potential customers," I guess. I'm familiar with onboarding as a term for what you do to new hires and was quite confused by this article until I figured out it's about marketing emails.
It's primarily used in the context of reaching out to users who just signed up for a service.
After they're signed up/subscribed, you send an "onboarding" email with general information on how to get started using the service, and encouraging them to do things that affect whatever KPIs your boss cares about.
They can be genuinely helpful for tools that have a steep learning curve, either by serving as an entry point into the documentation or by providing contact information for support.
Otherwise, at best they serve as a reminder in case you get distracted before you finish setting up your account.
