Empty trash on exit: The trash folder is cleared of messages when you quit KMail if this option is selected.
Send Mail in outbox Folder on Check: All messages queued in the outbox folder will be sent whenever you check your mail.
Automatically send receive- and read confirmations: With this option, a confirmation is automatically send to the sender of a message if he requested that. If you disable this option, no confirmation will be sent.
Compact all folders on exit: This will compact all folder every time you exit KMail. Compacting folders means that the messages you removed from a folder will really be removed.
Use external editor instead of composer: If you don't like the Composer you can use any editor. Note that the Composer window will still open and the external editor will open as soon as you type just one character in the body of the message.
New Mail Notification: This let's you configure actions for the arrival of new mail. Note that automatically fetching mail can be configured in your account settings under Receiving Mail.