This is the Gmail mute feature, which has been around since 2006. I guess you typed the letter m while reading a message that was not sent to you directly, e.g. from a mailing list. If you subscribe to a mailing list and you receive each new message in Gmail, you may run into boring threads that just continue to be boring. Gmail lets you mute these threads, so you still receive new messages but they're automatically archived.
To mute a thread, just type m while reading it. Each muted thread gets the label mute (and also muted), so you can find all your muted threads by searching for label:mute.
Note that this works only for mails that are not sent directly to you, which is the case for mailing lists.