You can add a comment to a segment in the lower-right part of the screen under Comments and Changes. After selecting a segment in the context view, start typing in the yellow Add comment... field and click Add when you're done. After this, you can also edit and delete the comments you've left in the file. Note that you may or may not be able to delete comments left by other users - this depends on the project settings.
In this same part of the screen, you will also see the change history for a segment, and you can choose to revert the translation to its previous version. You do this by highlighting the entry you want to revert to and clicking Apply.
