Sprint Retrospective: The Scrum Master participates as a peer team member - What's the meaning of this?

Yes? Why not?

If the dev team has identified an impediment from the SM, every member has to tell the SM about that. When : Ideally during the retrospective.

For many Scrum events, the Scrum Master is not a participant. Each event includes the different roles at different levels of participation including non-participation. During Sprint Planning the Scrum Master may help facilitate the meeting and provides input into the Sprint Goal; not much involvement. During the Daily Scrum the Scrum Master does not participate and may observe in order to later provide feedback to the Development Team on how to possibly improve the event. During the Sprint Review the Scrum Master may help facilitate the meeting. During the Sprint Retrospective the Scrum Master is an event participant at the same level as the Product Owner and Development Team members.

Since the Sprint Retrospective is for the entire Scrum Team to inspect and adapt how it does its work, the Scrum Master must be a peer participant. There needs to be a feeling of safety to promote candor during the inspect and adapt discussion. If the Scrum Master is in charge of the event or just an observer, then there is a good chance that the objectives will not be met.

HTH

> In our Retrospectives the Scrum Master is only the moderator
> of the Sprint Retrospective.
Is this wrong?
> When he should be more involved, isn't it difficult to moderate it?

Isn't your Scrum Master an authority on how to best implement the Scrum Framework? Why would such a person be disqualified from giving advice and opinion during Sprint Retrospectives?

And lets not forget the Product Owner as well!

Interesting, this sentence and this pharse "peer team member" is no longer part of the Scrum guide. Hard to "know" this in an exam, when the guide doesn't say it at all.

The Scrum Guide states that the entire Scrum Team participates in the Sprint Review, Sprint Retrospective and Sprint Planning.  This is the entire section that describes the Sprint Retrospective from the current Scrum Guide

The purpose of the Sprint Retrospective is to plan ways to increase quality and effectiveness.

The Scrum Team inspects how the last Sprint went with regards to individuals, interactions, processes, tools, and their Definition of Done. Inspected elements often vary with the domain of work. Assumptions that led them astray are identified and their origins explored. The Scrum Team discusses what went well during the Sprint, what problems it encountered, and how those problems were (or were not) solved.

The Scrum Team identifies the most helpful changes to improve its effectiveness. The most impactful improvements are addressed as soon as possible. They may even be added to the Sprint Backlog for the next Sprint.

The Sprint Retrospective concludes the Sprint. It is timeboxed to a maximum of three hours for a one-month Sprint. For shorter Sprints, the event is usually shorter.

Notice that the Scrum Team participates in the activities. Nowhere does it state that the Scrum Master's participation is limited to facilitation.  

You are correct that the phrase "peer team member" is not used anymore. But because of the way the guide describes and references Scrum Team, I don't see why it should be.  The entire guide focuses on inclusion of the entire team in decisions and activities.  The only exception is for the Daily Scrum which is for the Developers to discuss and plan the day's activities.