For example, you might have some corrections to point out on your or others' existing papers. Simply request a Commentary page, wait for activation, and then post a Comment.
For more details, visit the Commentaries how-to.
First, make sure the system has recognized you as an Author: go to your Personal Page and look at the Commentaries section; potential authorships are auto-detected, but you need to claim them by following the Claim Authorships link. After verification by an Editor, you will be enabled to reply as as Author for this and all other Comments on that particular page.
If you are registered as a Contributor, you can express your opinion by clicking on one of the coloured boxes next to the Comment (Agree, Not sure, Disagree). Any earlier opinion you had expressed gets erased. Aggregated opinions are given, and you cannot give an opinion on your own Comments.
Similar to Commentaries, Theses is a searchable repository of links to Master's, PhD and Habilitation theses (often held in a local institutional repository).
After requesting the Thesis Link, a page will be opened with the summary and external link, and the possibility to post Comments.