Getting started:

• Where do I get my php/html files?
• What type of files can I upload?
• What versions of php are supported?

About the site:

• How much does it cost?
• How secure are my files?
• Is it production safe?
• How does it work?
• Coming features

About the optimizations:

• Print statements into echo statements
• Double quotes into single quotes
• Period concatenation into comma concatenation
• Relative paths into direct paths
• Optimize tag attributes
• Remove unnecessary attributes

About the warnings:

• You may have javascript before style information
• Global variables copied to local variables
• MySQL queries inside loops
• MySQL queries that might be selecting more than necessary
• Image tags that are missing one or more dimensions
• Stylesheets possibly outside of the HEAD
• Javascript possibly in the HEAD
• Internal stylesheets
• Internal javascript
• Duplicate javascript files possibly
• Duplicate stylesheets possibly
• Duplicate javascripts possibly

After your file is uploaded and secured, the processor systematically goes through your code and makes the necessary changes. Other than that it's pretty hard to explain.