Syntax: @skip [<collection>...]
For each collection of mail messages that you read other from your own, a current message and a last-read-time is kept. Normally, if you neglect to actually @read any messages on a collection other from your own, @rn (or :check_mail_lists) will continue to remind you that they are there.
The @skip command indicates that you're not interested in reading the rest of the messages that currently exist in that given collection.
The @unsubscribe command flushes the current-message/last-read-time information completely, indicating that you are not only uninterested in the rest of the messages on that collection, but also likewise uninterested in anything else that may appear on that collection later on. @unsubscribe also removes you from the collection's .mail_notify list.
See also: help @subscribe