SPF stands for Sender Policy Framework. It helps verify that emails are sent from authorized servers.
When an email is sent, the sending server adds information to the email to prove it came from a legitimate source. The receiving server checks this information against the "SPF record" that lists which servers are allowed to send for that domain.
If the sending server is not listed in the SPF record, there is a "violation" in that the receiving server couldn't verify the email actually came from a trusted source. To block potential spoofing and phishing, many servers will reject emails that fail SPF checks.