This page is not meant to be interpreted as a legal document. It explains what data is collected where in everyday plain English.
When the "Collect usage data" option is enabled (on by default), anonymized usage data is reported for analytics purposes. This includes actions the user took and their preferences. Any information that could identify a person is removed before it's sent, a random client ID is generated and reported. This data helps the developers understand how the extension is used. If you want the exact details of what is collected and reported check the source code.