One of the Enhanced Features (see the rest here) is Lead Verification.

We have 3rd party data providers that pull profile data associated with a new lead's email and phone number. If the profile data matches across the various data profiles then we have high confidence that the lead has provided accurate info to you.
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Verified Leads have a blue outline around their avatar in the My Leads table. If you open the lead's details, you'll see a "Verified" badge below their photo too.

Leads that aren't verified aren't necessarily fake though. Just means that we're less confident in their data.