SpamAssassin Rule: <RCVD_IN_PBL>
Standard description: Received via a relay in Spamhaus PBL
The PBL official page and description is here and that page can also be used to look up an IP in the PBL.
The Spamhaus PBL is a DNSBL database of end-user IP address ranges which should not be delivering unauthenticated SMTP email to any Internet mail server, except those provided for specifically by an ISP for that customer's use. The PBL helps networks enforce their Acceptable Use Policy for dynamic and non-MTA customer IP ranges.
In other words, it lists IP addresses that are not expected to host a normal mail server, and the PBL has Self-Service removal and IP owner management mechanisms.
The default scores for this rule can be found in the online list of tests.