Audit is a predefined set of factors that compares top 5 pages* to any page you want. It will tell you where you are off and how to fix it.
*you can compare how many pages you want. Lear more about excluding pages from analysis:
Audit consists of a segments which group factors:
True Density (click to read more)
page structure (number of elements),
True density is the most important thing in audit. It will tell you which terms you are missing as well as over-optimized. The first actionable task to start optimization is there.
Surfer audit will warn you or show an error when audited page is way off the competitors pages. Fix your warnings and errors to raise performance of your content.