Tmux is a terminal multiplexer that allows you to create and control sessions. It is often used to run a command in the background and be able to close the terminal without stopping the process. When you start Tmux, it initiates a session stored (by default) in your /tmp directory. By closing the window, reopening a terminal, and running tmux attach, you can retrieve your processes. It is also possible to open multiple Tmux sessions at the same time.