Tmux commands: What is M-whatever

It's the meta key. So M-1 is meta-1. (Just like how C-1 is control-1).

Now, when you look at your keyboard, you probably notice the distinct lack of any key actually labeled meta, at least if you have a normal PC keyboard. Depending on how your keyboard layout is set up, meta is typically either the alt key or the logo (Windows) key.

In short, C-a M-1 is telling you to press and hold Control and press A; then release both; then press and hold Alt (or Windows) and press 1. The release them, of course.

Years later, I came across

Notice the Meta key to the left and right of the keyboard! Along with a host of other useful keys. I wonder what Rub Out does...