When you open a meeting request letter, the Event Reply buttons are displayed:
If you click the Accept or Tentative button, a positive reply is sent back to the Event organizer, and the Event is copied into your Main Calendar folder (Mailbox).
If you click the Decline button, a negative reply is sent back to the Event organizer, and the Event is not stored in your Main Calendar folder.
You may want to enter a comment into the panel text field.
If you click any of the Accept/Tentative/Decline buttons, the original request letter is deleted.