The prefix key for tmux is a key that tells an active tmux session that the following key should be interpreted as a tmux command instead of regular input to the terminal.

By default, tmux has Ctrl-b bound to the prefix.

So, hitting Ctrl-b followed by ? will open up the tmux help.

Typing Ctrl-b involves a bit of a finger stretch. It can be changed to something more comfortable. My personal preference is Ctrl-z. Many configurations I've seen use Ctrl-a.

To change this, open up your ~/.tmux.conf file and add two lines.

unbind C-b
set -g prefix C-z

And then run :source-file ~/.tmux.conf.

This tells tmux to unbind Ctrl-b so that it no longer is interpreted as the prefix, and then it binds Ctrl-z as the new prefix key.