Unix: what modifier key does "M-" refer to (e.g. M-C)

Esc

For instance, if you wanted to go to the end of the file press

Esc then /

You don't need to hold down Esc as if it were Shift.

I do not know for nano, but for emacs, M- stands for the Meta key, which can mean pressing the Alt key simultaneously with the letter key or by pressing Esc key before the letter key. Alt generally only works when using an X version of the application, so when using a dumb terminal, only Esc will work.

M refers to the meta key, which is not present on most keyboards. (Its use in software is for primarily historical reasons.) Usually, the meta key is emulated by another key on your keyboard. On Windows and Linux, it is usually the Alt key. On Mac OS X, that key (aka Option) already has other uses, and so Escape is used instead.