tmux is a terminal multiplexer. It allows you to run multiple command line commands on a single terminal. Never again will you need to open several terminals to execute a task or keep going back and forth between directories.
tmux comes out of the box in some distributions. You can also install it by using most of the package management tool. There is the option to compile it yourself as well. Check out its installing wiki for more details.
The tmux getting started states the main uses of tmux are:
Developer, testable code advocate and command line tools enthusiast