If you want To ignore a thread, you need to right-click on it and select , or you can use K [for “kill”] keyboard shortcut. That will mark all messages in the selected conversation as ignored. The ignored messages aren’t deleted, but once you come back to the folder they won’t be shown on the list of messages (by default). If there are further replies to the same conversation, these replies won’t trigger a new mail notification.
Ignoring subthreads is similar to ignoring threads, but ignores only replies from the selected message onwards. This can be used to ignore otherwise interesting conversations that get off topic from a certain response. To ignore a subthread, you need to right-click the first of the messages you would like to ignore and select .