If you hover over these symbols, you'll see little pop-ups with explanations.
1) The "pennant" is a flag you can mark any message with for reasons which are entirely up to you. Usually it is used to mark messages you consider important
2) The X means "delete"
3) envelope+arrow means "make read/unread"
And, finally, to open the message, click anywhere on the text in the Subject or From columns.