what mean is M-Up key?

[John Wong]

Sorry, the question maybe ......
I'm tmux/openbsd user, i read the man page, and see some key is like "M-Up
resize-pane -U 5".
But what exactly should i type?

I know C-b mean is "Ctrl + b", but M-o ...
what is "M-" key?
Please help, thank you.