tmux

tmux is a "terminal multiplexer", meaning its main functionality is in splitting terminal windows and controlling multiple terminals from the same window. In practice, I use it to detach a terminal window from the window itself (e.g. putting cmus in a tmux server and then detaching means the music still plays, even if the window is closed. The music player can then be brought up again with tmux attach, if I want to switch songs or adjust the volume). These days I don't make much use of tmux anymore, instead preferring to use features from my window manager, terminal emulator, or text editor.

tmux is probably most helpful for remote connections with ssh, so your commands keep running even after you close the connection.