You can star messages to indicate that they're important to you. To mark a conversation with a star, click its star icon in the conversation list. You can star an individual message by clicking the star at the upper right of the message itself.
With Gmail accounts, starred messages appear in the Starred folder in the folder list.
Geary marks messages as read automatically as you read them. To manually toggle a conversation as read or unread, click the circle icon in the conversation list.
Alternately, the Mark as Unread in the Mark menu on the toolbar can be used to toggle the read status of the selected conversation(s).
To mark an individual message as read, select Mark as Read from the dropdown menu.