What does Muted mean in terms of GMAIL?

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.

Interesting considering this was an email directly sent to me from a friend in England. How can I "unmute" it?

Select the conversation you want to unmute (demute?), and from the drop-down choose "Move to inbox."

It's not giving me that option as I guess it's still in my Inbox already. I wish Gmail had a valid customer tech support.

Move the message to another folder (you can create one) and then choose to view "All Mail"; when you highlight the muted message or conversation, the option to move it to the Inbox will become visible, this will unmute it.

would if i sent i message out but didn't mean to.... could mute fix that?

If you mean you want to recall a Gmail message, you cannot.