SpamAssassin will analyze your incoming email, and give it a score indicating how spammy the email might be. It looks at a variety of factors and tries to estimate the likelihood that an email is spam or not. Many email clients then use this score to determine how to display the email to you. For example, emails with a high spam score may go directly into the spam folder or be deleted.
How to use Apache SpamAssassin to control unwanted email in cPanel.