tmux is a "terminal multiplexer: it enables a number of terminals to be created, accessed, and controlled from a single screen in Linux. tmux may be detached from a screen and continue running in the background, then later reattached."
tmux is my favorite terminal multiplexer, a program that lets you run many applications in the same terminal, with one keystroke to switch. tmux is very powerful and flexible. You can resize panes, set up shortcuts for launching programs, even use it to create text macros.
There's a course on tmux here, that explains how to use it.
In other words, it makes working on multiple programs at the same time simpler.
A tmux session consists of one or more windows and zero or more panes. A window contains a single program; zero or more panes occupy each window. Each pane takes up one row of the window and has one or more lines of text.
tmux is one of those programs that I can't live without as a programmer. I use it every day, multiple times a day. It's just so efficient in running my multiple programming sessions.
The only problem is that tmux isn't available for Windows, and if you're a programmer, you spend most of your time in your terminals anyway.
In order to use tmux on Windows, you have to install a virtual machine on top of your existing operating system. This works - but it's completely unnecessary. But the Windows terminal is very limited anyhow