This is a terminal multiplexer, which means that you can use it to "spawn" multiple terminals. You can see it as "tabs" on your web browser. They're all "linked" to the same app, but allow performing different tasks.

That's the terminal multiplexing part. Over it, tmux provide another feature: the panes. You can split the tmux display into multiple parts, in order to display more than one terminal at the same time.