The term "spam analysis" refers to a feature of the test email campaign process (Step 5 of the Create a Campaign Workflow).
When you select to run a spam analysis on your email campaign, the system will run your email through SpamAssassin to check against common spam filters. It scans your email for trigger words, large images, and blacklisted URLs. The analysis report is sent back to you when complete as a text-only email with a score. You can also find the report in the Create a Campaign test step under "View Test Send History."