screen and tmux have a copy mode (this is "inside" of the terminal emulator, I'm not talking about using your mouse to select text in Terminal.app) that allow you to copy text. I assume he wants copy there to yank the text to the OS X clipboard.
It's possible in tmux, but kind of annoying (the best solution I've found requires another key combo press to push your copy into the system clipboard).
You can select stuff with the mouse in the terminal and whatever you select is then available to be pasted with CMD+V.
What the parent refers to is the ability to press C-b to enter the buffer scroll, then select text there and unfortunately -- not -- having that available to OSX's paste buffer, but only internally inside tmux.
This means that if you want to copy that stuff to OSX's paste buffer you have to go through the intermediate step of pasting it inside a new buffer (say, Vim) inside tmux, then select it again with the standard terminal selection feature and only then it will be available to OSX's CMD+V.
