When the "Collect usage data" option is enabled (on by default), anonymized usage data is reported for analytics purposes. This includes user actions (for example the user has viewed their inventory or market listings, sorted their items, generated a list of their items) and the extension preferences they set (any potential identity revealing preferences like customization is only reported as "custom"). The extension is only allowed to run on a very limited set of pages, it does not have permission to collect a users general browsing data.
Any information that could identify a person is removed before it's sent, a random client ID is generated and reported. The analytics collection solution is a first party service. It helps the developers understand how the extension is used and is not shared with any third parties. An aggregated form of this data may be displayed publicly in a dashboard format.