Typically, you would connect to your server with an SSH client like PuTTY and after logging in you are dropped into a shell like Bash where you can enter commands and interact with the operating system. For most purposes, this simple setup suffices. However, in certain scenarios, you may need, or at least benefit, from expanding the capabilities of such a session with a terminal multiplexer like tmux. Just like the operating system running on your phone or computer can execute and display, side by side, multiple applications on its graphical user interface, so can tmux do with text-based programs and shell sessions.