tmux is a terminal multiplexer for Unix-like operating systems. It allows multiple terminal sessions to be accessed simultaneously in a single window. It is useful for running more than one command-line program at the same time.
To launch tmux, simply type tmux on the terminal. If you encounter an error, you can add /usr/bin to your path, e.g. export PATH=/usr/bin:$PATH, or use an alias, e.g. alias tmux=/usr/bin/tmux (run the command or add to your ~/.bashrc).