tmux (terminal multiplexer) is a terminal (text-based) sessions manager. It's most useful for managing text sessions in remote SSH sessions. It's a greatly customisable and wonderfully simple to use software – but as with many great things there's a steep enough learning curve.
Current version: 2.9
Also available: tmux 3.0 rc4
How tmux looks
This shows a typical tmux session: you have 1 window named bash (seen in blue highlight at the bottom of the screen on the left) and two panes in that window – each is a separate terminal session: